Radiation source and lithographic apparatus

ABSTRACT

A radiation source comprising a fuel source configured to deliver fuel to a location from which the fuel emits EUV radiation. The radiation source further comprises an immobile fuel debris receiving surface provided with a plurality of grooves. The grooves have orientations which are arranged to direct the flow of liquid fuel under the influence of gravity in one or more desired directions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application61/663,210, which was filed on 22 Jun. 2012, U.S. provisionalapplication 61/679,567, which was filed on 3 Aug. 2012, U.S. provisionalapplication 61/702,443 which was filed on 18 Sep. 2012, U.S. provisionalapplication 61/713,922, which was filed on 15 Oct. 2012, U.S.provisional application 61/722,488, which was filed on 5 Nov. 2012, U.S.provisional application 61/739,358, which was filed on 19 Dec. 2012,U.S. provisional application 61/806,644, which was filed on 29 Mar. 2013and which are incorporated herein in their entirety by reference.

FIELD

The present invention relates to a radiation source and to alithographic apparatus.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.,comprising part of, one, or several dies) on a substrate (e.g., asilicon wafer). Transfer of the pattern is typically via imaging onto alayer of radiation-sensitive material (resist) provided on thesubstrate. In general, a single substrate will contain a network ofadjacent target portions that are successively patterned.

Lithography is widely recognized as one of the key steps in themanufacture of ICs and other devices and/or structures. However, as thedimensions of features made using lithography become smaller,lithography is becoming a more critical factor for enabling miniature ICor other devices and/or structures to be manufactured.

A theoretical estimate of the limits of pattern printing can be given bythe Rayleigh criterion for resolution as shown in equation (1):

$\begin{matrix}{{C\; D} = {k_{1}*\frac{\lambda}{N\; A}}} & (1)\end{matrix}$where λ is the wavelength of the radiation used, NA is the numericalaperture of the projection system used to print the pattern, k1 is aprocess dependent adjustment factor, also called the Rayleigh constant,and CD is the feature size (or critical dimension) of the printedfeature. It follows from equation (1) that reduction of the minimumprintable size of features can be obtained in three ways: by shorteningthe exposure wavelength λ, by increasing the numerical aperture NA or bydecreasing the value of k1.

In order to shorten the exposure wavelength and, thus, reduce theminimum printable size, it has been proposed to use an extremeultraviolet (EUV) radiation source. EUV radiation is electromagneticradiation having a wavelength within the range of 5-20 nm, for examplewithin the range of 13-14 nm. It has further been proposed that EUVradiation with a wavelength of less than 10 nm could be used, forexample within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Suchradiation is termed extreme ultraviolet radiation or soft x-rayradiation. Possible sources include, for example, laser-produced plasmasources, discharge plasma sources, or sources based on synchrotronradiation provided by an electron storage ring.

EUV radiation may be produced using a plasma. A radiation system forproducing EUV radiation may include a laser for evaporating or excitinga fuel to provide the plasma, and a radiation source for containing theplasma. The plasma may be created, for example, by directing a laserbeam at a fuel, such as particles of a suitable material (e.g., tin), ora stream of a suitable gas or vapor, such as Xe gas or Li vapor. Theresulting plasma emits output radiation, e.g., EUV radiation, which iscollected using a radiation collector. The radiation collector may be amirrored normal incidence radiation collector, which receives theradiation and focuses the radiation into a beam. The radiation sourcemay include an enclosing structure or chamber arranged to provide avacuum environment to support the plasma. Such a radiation system istypically termed a laser produced plasma (LPP) source. In an alternativeradiation system, the plasma is generated by applying an electricaldischarge across a gap at which fuel such as tin is located. Such aradiation system is typically termed a discharge produced plasma (DPP)source.

Vaporization of the fuel which generates the plasma may be incomplete,and droplets of un-vaporized fuel may therefore be incident uponsurfaces of the radiation source. Accumulation of fuel on opticalsurfaces of the radiation source may be undesirable because the fuelwill modify the reflectivity of the optical surfaces.

SUMMARY

It is desirable to reduce the accumulation of fuel on one or moreoptical surfaces of a radiation source in a manner which is not knowfrom the prior art.

According to first aspect of the invention, there is provided aradiation source comprising a fuel source configured to deliver fuel toa location from which the fuel emits EUV radiation, wherein theradiation source further comprises an immobile fuel debris receivingsurface provided with a plurality of grooves, the grooves havingorientations which are arranged to direct the flow of liquid fuel underthe influence of gravity in one or more desired directions.

The fuel debris receiving surface may be provided with a plurality ofvanes, the plurality of grooves being provided in the vanes.

At least some of the grooves may have a cross-sectional size and/orshape which gives rise to capillary action.

At least some of the grooves may have a cross-sectional size and/orshape which gives rise to wicking action which draws liquid fuel intothe grooves.

One or more of the grooves may include a corner which extendslongitudinally along the groove.

One or more of the grooves may be v-shaped in cross-section.

The v-shaped groove may have an opening angle which is between around30° and 50°.

The grooves may comprise a set of grooves which extend substantiallyparallel to one another.

At least some of the grooves may have a depth of 0.1 mm or more.

At least some of the grooves may have a depth of 2 mm or less.

At least some of the grooves may have a width of 0.1 mm or more.

At least some of the grooves may have a width of 10 mm or less.

Adjacent grooves may be separated by a distance which is equal to orless than twice the capillary length of the liquid fuel.

The vanes may be distributed around a housing of the radiation source.

The vanes may be reflective structures located in the vicinity of anintermediate focus of the radiation source.

The vanes may be located in a fuel catcher of the radiation source.

One or more of the vanes may be substantially helical.

The fuel may be tin, xenon or lithium.

The vanes may be heated to a temperature at which is above the meltingtemperature of the fuel. The vanes may be heated to a temperature whichis below the evaporation temperature of the fuel.

According to a second aspect of the invention there is provided anapparatus comprising the radiation source of the first aspect of theinvention, such as a lithographic apparatus, a substrate (e.g. wafer,mask) inspection apparatus, a contamination cleaning apparatus, asubstrate processing apparatus or a calibration apparatus.

Optionally, a lithographic apparatus may further comprise one or more ofan illumination system configured to condition EUV radiation receivedfrom the radiation source, a support constructed to support a patterningdevice, the patterning device being capable of imparting the EUVradiation with a pattern in its cross-section to form a patternedradiation beam, a substrate table constructed to hold a substrate, and aprojection system configured to project the patterned radiation beamonto a target portion of the substrate.

A substrate inspection apparatus may comprise one or more of thefollowing elements: an extreme ultraviolet light source; a support forreceiving a substrate for inspection; an extreme ultraviolet imagingsensor; illumination optics located between the extreme ultravioletlight source and the support for the substrate; objective optics locatedbetween the support for the substrate and the imaging sensor; and aspectral purity filter disposed on or located proximate to the imagingsensor having one or more selected spectral characteristics.

The contamination cleaning apparatus may further comprise a gas flow, areactive gas, an electrical field, a vibration inducing actuator orother means to clean undesired fuel debris from a contaminated surface.

According to a third aspect of the invention there is provided a methodof generating EUV radiation using a radiation source, the methodcomprising delivering fuel to a location at which a plasma which emitsEUV radiation is generated using the fuel, wherein the method furthercomprises receiving liquid fuel on an immobile surface of the radiationsource, and using grooves provided in the immobile surface to direct theflow of liquid fuel under the influence of gravity in one or moredesired directions.

According to a fourth aspect of the invention there is provided aradiation source comprising a fuel source configured to deliver fuel toa location from which the fuel emits EUV radiation, wherein theradiation source further comprises a fuel debris receiving surface, thefuel debris receiving surface being connected to a liquid inlet which isconfigured to deliver liquid alloy or metal onto the fuel debrisreceiving surface. A plurality of grooves with characteristics asdescribed above for the first aspect of the invention, the grooveshaving orientations which are arranged to direct the flow of liquid fuelunder the influence of gravity in one or more desired directions, mayalso be provided in this aspect of the invention.

According to a fifth aspect of the invention there is provided aradiation source comprising a fuel source configured to deliver fuel toa location from which the fuel emits EUV radiation, wherein theradiation source further comprises a fuel debris receiving surface and aliquid inlet configured to deliver a flow of liquid alloy or metal ontothe fuel debris receiving surface. A plurality of grooves withcharacteristics as described above for the first aspect of theinvention, the grooves having orientations which are arranged to directthe flow of liquid fuel under the influence of gravity in one or moredesired directions, may also be provided in this aspect of theinvention.

The liquid inlet may be connected to the fuel debris receiving surface.

The liquid inlet may be configured to provide a coating of liquid alloyor metal on the fuel debris receiving surface.

The fuel debris receiving surface may comprise a plurality of vanes.

Spaces between the vanes may be grooves which direct the flow of liquidalloy or metal under the influence of gravity in one or more desireddirections.

The liquid inlet may comprise openings located in the grooves.

The liquid inlet may be configured to deliver liquid alloy or metal at arate that fills an area at the base of the vanes to a desired filllevel.

The vanes may be shaped to generate capillary pressure which drawsliquid alloy or metal away from tips of the vanes.

The liquid inlet may comprise a plurality of openings connected to aconduit.

The liquid alloy or metal may be liquid fuel.

The radiation source may further comprise a heater configured to heatthe fuel debris receiving surface to a temperature which is above themelting temperature of the fuel.

The liquid inlet may comprise a porous metal through which the metal oralloy is delivered.

The liquid inlet may be configured to deliver a metal or alloy which isliquid at room temperature onto the fuel debris receiving surface.

The metal or alloy may be Galinstan. Alternatively, the metal or allowmay be tin or another tin alloy or a further liquid fuel suitable toproduce EUV radiation.

The radiation source may comprise a cooling apparatus configured to coola housing of the radiation source.

The cooling apparatus may be configured to cool the housing of theradiation source to around room temperature.

According to a sixth aspect of the invention there is provided aradiation source comprising a fuel source configured to deliver liquidalloy or metal to a location from which the fuel emits EUV radiation,wherein the radiation source further comprises a vane and an inletconfigured to deliver liquid alloy or metal to the vane and therebymaintain a coating of liquid alloy or metal on the vane. A plurality ofgrooves with characteristics as described above for the first aspect ofthe invention, the grooves having orientations which are arranged todirect the flow of liquid fuel under the influence of gravity in one ormore desired directions, may also be provided in this aspect of theinvention.

According to a seventh aspect of the invention there is provided amethod of controlling contamination in a radiation source which usesfuel to generate EUV radiation, the method comprising delivering liquidalloy or metal via an inlet onto a fuel debris receiving surface suchthat a coating of liquid alloy or metal is maintained on the fuel debrisreceiving surface.

In the method of the sixth or seventh aspect of the invention, the alloyor metal may be delivered continuously via the inlet.

In the method of the sixth or seventh aspect of the invention, the alloyor metal may be delivered intermittently via the inlet.

In the method of the sixth or seventh aspect of the invention, the inletmay comprise a porous metal through which the metal or alloy isdelivered.

In the method of the sixth or seventh aspect of the invention, the metalor alloy may be liquid at room temperature.

In the method of the sixth or seventh aspect of the invention, the metalor alloy may be Galinstan.

In the method of the sixth or seventh aspect of the invention, themethod may further comprise cooling a housing of the radiation source toroom temperature.

According to an eighth aspect of the invention, there is provided aliquid fuel debris guiding apparatus comprising a surface, twoelectrodes separated from the surface by an insulating layer. A gap isprovided between the two electrodes that defines a path on the surface,and a voltage source configured to apply a voltage to one of theelectrodes, thereby establishing a potential difference across the gapbetween the electrodes. The potential difference acts to guide liquidfuel droplets along the path defined by the gap.

The apparatus may further comprise one or more additional electrodesconnected to one or more voltage sources, gaps being provided betweenthe electrodes to define paths on the surface.

According to a ninth aspect of the invention there is provided a methodof directing a flow of liquid fuel debris. The method comprises applyinga voltage to one of two electrodes that are separated from the surfaceby an insulating layer. A gap is provided between the two electrodes todefines a path on the surface. The voltage establishes a potentialdifference across the gap between the electrodes, which acts to guideliquid fuel droplets along the path defined by the gap.

According to a tenth aspect of the invention there is provided fuelcollector for an EUV radiation source, the fuel collector comprising areceptacle and a reservoir, the reservoir being located above thereceptacle, wherein the reservoir is provided with a hole through whichliquid fuel may drain from the reservoir into the receptacle, andwherein a raised lip extends around the hole, the raised lip preventingliquid fuel from passing into the hole until a level of the liquid fuelexceeds the height of the raised lip. Such fuel collector may be usedtogether with any radiation source according to the aspects of theinvention described herein, to further enhance the debris mitigation.

The raised lip may be formed from a non-wetting material.

The raised lip may be formed from molybdenum.

The raised lip may have a rounded upper surface.

The height of raised lip may be equal to or greater than a capillarylength of the liquid fuel.

The fuel collector may further comprise a lip which projects downwardlyfrom the hole.

The downwardly projecting lip may have a sharp inner corner.

According to an eleventh aspect of the invention there is provided aradiation source configured to deliver fuel to a location from which thefuel emits EUV radiation, wherein the radiation source comprises a fueldebris receiving surface and a fuel collector, the fuel collectorcomprising a receptacle and a reservoir, the reservoir being locatedabove the receptacle, wherein the reservoir is provided with a holethrough which liquid fuel may drain from the reservoir into thereceptacle, and wherein a raised lip extends around the hole, the raisedlip preventing liquid fuel from passing into the hole until a level ofthe liquid fuel exceeds the height of the raised lip.

According to a twelfth aspect of the invention there is provided aradiation source housing apparatus comprising a rotatably mountedhousing, an actuator arranged to drive the housing to rotate, a heaterlocated adjacent to a first portion of the housing, and a cooler locatedadjacent to a second different portion of the housing. A plurality ofgrooves with characteristics as described above for the first aspect ofthe invention, the grooves having orientations which are arranged todirect the flow of liquid fuel under the influence of gravity in one ormore desired directions, may also be provided in this aspect of theinvention.

Optionally, the first portion of the housing does not have a downwardlyfacing inner surface.

Optionally, the first portion of the housing does not have an innersurface from within which liquid fuel can drip.

The first portion of the housing may be a lowermost side of therotatably mounted housing.

The heater may be arranged to heat the first portion of the rotatablymounted housing to a temperature which is above the melting temperatureof tin, and the cooler may be arranged to cool the second portion of therotatably mounted housing to a temperature which is below the meltingtemperature of tin.

The cooler may extend around at least two thirds of the circumference ofthe rotatably mounted housing.

The heater may extend around less than one third of the circumference ofthe rotatably mounted housing.

Optionally, the heater and the cooler do not overlap around thecircumference of the rotatably mounted housing.

Optionally, the heater does not extend so far around the housingcircumference that the tin will remain in liquid form after the housinghas rotated to a point at which the tin is on a downwardly facingsurface.

The inner surface of the housing may be provided with grooves.

According to a thirteenth aspect of the invention there is provided aradiation source comprising a fuel source configured to deliver fuel toa location from which the fuel emits EUV radiation, and furthercomprising a radiation source housing apparatus comprising a rotatablymounted housing, an actuator arranged to drive the housing to rotate, aheater located adjacent to a first portion of the housing, and a coolerlocated adjacent to a second different portion of the housing.

The heater may be arranged to heat the first portion of the rotatablymounted housing to a temperature which is above the melting temperatureof the fuel, and the cooler may be arranged to cool the second portionof the rotatably mounted housing to a temperature which is below themelting temperature of the fuel.

The heater may be arranged to heat the first portion of the rotatablymounted housing to a temperature at which fuel on the housing will be ina liquid state, and the cooler may be arranged to cool the secondportion of the rotatably mounted housing to a temperature at which fuelon the housing will be in a solid state.

According to a fourteenth aspect of the invention there is provided amethod of generating EUV radiation comprising delivering fuel to alocation from which the fuel emits EUV radiation, the method furthercomprising driving a radiation source housing to rotate, heating aportion of the housing to a temperature which is above the meltingtemperature of the fuel, and cooling a portion of the housing to atemperature which is below the melting temperature of the fuel.

According to a fifteenth aspect of the invention there is provided afuel collector for an EUV radiation source, the fuel collectorcomprising: a receptacle, said receptacle being provided with anentrance and a storage portion; an object, which is disposed within thereceptacle such that fuel passing through the entrance is incident upona surface of the object; and a fuel transferring mechanism configured totransfer fuel collected upon the surface to the storage portion. Suchfuel collector may be used together with any radiation source accordingto the aspects of the invention described herein, to further enhance thedebris mitigation.

Such an arrangement allows quantities of fuel to be deposited in thestorage portion periodically. Advantageously, this can prevent theformation of a single stalagmite in the receptacle and lead to animprovement in the filling rate of receptacle.

The temperature of the object may be below the melting point of thefuel. The temperature of the object may be such that liquid fuelincident upon the object is allowed to solidify before being transferredto the storage portion.

The fuel transferring mechanism may be arranged to transfer fuelcollected upon the surface to the storage portion periodically. Fuel maybe transferred to the storage portion after a predetermined period oftime or, alternatively, when a sufficient quantity of fuel has beenincident upon the object.

The object may be formed from a low wetting material. In particular, theobject may be formed from a material which is low wetting with respectto a fuel it is desired to collect. The fuel collector may beparticularly suitable for the collection of tin and the object may beformed from molybdenum.

The object may be operable to move between a first position and a secondposition. Movement of the object between the first and second positionsmay provide the fuel transferring mechanism. For example, when theobject is disposed in the second position, any fuel deposited thereonmay fall under gravity into the storage portion. The object may bearranged so that as fuel is incident upon the surface, the object movesfrom the first position towards the second position.

The object may be resiliently biased towards the first position. Theobject may comprise a leaf spring and may provide its own resilientbias.

The object may comprise a cantilever structure.

The object may comprise a wheel, wherein rotation of the wheel providesthe fuel transferring mechanism.

The object may comprise a shelf, and a member that is operable to sweepacross the shelf may provide the fuel transferring mechanism.

The fuel collector may further comprise first and second valves arrangedto form an air lock.

According to a sixteenth aspect of the invention there is provided afuel collector for an EUV radiation source, the fuel collectorcomprising: a receptacle, said receptacle being provided with a surfacearranged such that fuel passing through an entrance of the receptacle isincident thereupon, wherein the surface is formed from a material whichis low wetting with respect to a fuel it is desired to collect, thetemperature of the surface is below the melting point of the fuel andthe surface is inclined with respect to horizontal such that the surfaceforms a slide. Such fuel collector may be used together with anyradiation source according to the aspects of the invention describedherein, to further enhance the debris mitigation.

Such an arrangement allows quantities of fuel to be deposited in astorage portion of the receptacle periodically. As liquid fuel isincident upon the surface it will solidify and will only weakly adhereto the surface. As a sufficient quantity of solid fuel builds up it willslide off the surface under gravity. Advantageously, this can preventfuel from adhering to the receptacle which allows the receptacle to bemore easily emptied.

The fuel collector may comprise one or more valves, which may be vacuumvalves.

A first valve may be disposed towards and entrance end of thereceptacle. During normal operation of the EUV source the first valvemay be open, allowing fuel to enter the receptacle. The first valve maybe closed periodically to isolate the receptacle from the EUV source soas to allow fuel to be removed therefrom.

The fuel collector may comprise an exit. This may allow fuel to beremoved from the receptacle. The exit may be disposed at a lower portionof the receptacle.

A second valve may be located at the exit of the receptacle. Duringnormal operation of the EUV source the second valve may be closed sothat the receptacle may be maintained at substantially the same pressureas the EUV source. The second valve may be opened periodically so as toallow fuel to be removed from the receptacle

The first and second valves may form an air lock.

Different features of different aspects of the invention may be combinedtogether where appropriate.

Further features and advantages of the invention, as well as thestructure and operation of various embodiments of the invention, aredescribed in detail below with reference to the accompanying drawings.It is noted that the invention is not limited to the specificembodiments described herein. Such embodiments are presented herein forillustrative purposes only. Additional embodiments will be apparent topersons skilled in the relevant art(s) based on the teachings containedherein.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the relevant art(s) to makeand use the invention.

FIG. 1 schematically depicts a lithographic apparatus according to anembodiment of the invention;

FIG. 2 is a more detailed schematic view of the lithographic apparatus;

FIG. 3 schematically depicts a radiation source according to anembodiment of the invention viewed from above;

FIG. 4 schematically depicts the radiation source of FIG. 3 viewed fromone side;

FIG. 5 schematically depicts a groove which may be formed in a surfaceaccording to an embodiment of the invention;

FIG. 6 is a graph which represents the effect of changing an anglesubtended by the groove;

FIG. 7 schematically depicts a tilted groove according to an embodimentof the invention;

FIG. 8 is a graph which represents the effect of changing an anglesubtended by the groove;

FIG. 9 is a graph which represents the effect of changing an angle atwhich the groove is tilted, and the effect of changing the depth of thegroove;

FIG. 10 schematically depicts a fuel catcher 22 according to anembodiment of the invention;

FIG. 11 schematically depicts part of a radiation source according to anembodiment of the invention;

FIG. 12 schematically depicts in more detail part of the radiationsource of FIG. 11;

FIGS. 13 and 14 are graphs which represent curvature of a vane shown inFIG. 11 and an obscuration bar shown in FIGS. 3 and 4, respectively;

FIG. 15 schematically depicts a liquid fuel guiding apparatus viewedfrom above;

FIG. 16 schematically depicts the liquid fuel guiding apparatus of FIG.15 in cross-section;

FIG. 17 schematically depicts the liquid fuel guiding apparatus of FIG.15 in operation;

FIG. 18 schematically depicts a fuel collector according to anembodiment of the invention which may form part of the radiation source;

FIG. 19 schematically depicts in partial cross-section part of aradiation source according to an embodiment of the invention;

FIG. 20 schematically depicts in cross-section the part of the radiationsource shown in FIG. 19;

FIG. 21 schematically depicts in cross-section a fuel collectoraccording to an embodiment of the invention; and

FIG. 22 schematically depicts in cross-section a fuel collectoraccording to an alternative embodiment of the invention.

The features and advantages of the present invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings, in which like reference charactersidentify corresponding elements throughout. In the drawings, likereference numbers generally indicate identical, functionally similar,and/or structurally similar elements. The drawing in which an elementfirst appears is indicated by the leftmost digit(s) in the correspondingreference number.

DETAILED DESCRIPTION

This specification discloses one or more embodiments that incorporatethe features of this invention. The disclosed embodiment(s) merelyexemplify the invention. The scope of the invention is not limited tothe disclosed embodiment(s). The invention is defined by the claimsappended hereto.

The embodiment(s) described, and references in the specification to “oneembodiment”, “an embodiment”, “an example embodiment”, etc., indicatethat the embodiment(s) described may include a particular feature,structure, or characteristic, but every embodiment may not necessarilyinclude the particular feature, structure, or characteristic. Moreover,such phrases are not necessarily referring to the same embodiment.Further, when a particular feature, structure, or characteristic isdescribed in connection with an embodiment, it is understood that it iswithin the knowledge of one skilled in the art to effect such feature,structure, or characteristic in connection with other embodimentswhether or not explicitly described.

Embodiments of the invention may be implemented in hardware, firmware,software, or any combination thereof. Embodiments of the invention mayalso be implemented as instructions stored on a machine-readable medium,which may be read and executed by one or more processors. Amachine-readable medium may include any mechanism for storing ortransmitting information in a form readable by a machine (e.g., acomputing device). For example, a machine-readable medium may includeread only memory (ROM); random access memory (RAM); magnetic diskstorage media; optical storage media; flash memory devices; electrical,optical, acoustical or other forms of propagated signals (e.g., carrierwaves, infrared signals, digital signals, etc.), and others. Further,firmware, software, routines, instructions may be described herein asperforming certain actions. However, it should be appreciated that suchdescriptions are merely for convenience and that such actions in factresult from computing devices, processors, controllers, or other devicesexecuting the firmware, software, routines, instructions, etc.

Before describing such embodiments in more detail, however, it isinstructive to present an example environment in which embodiments ofthe present invention may be implemented.

FIG. 1 schematically shows a lithographic apparatus 100 including aradiation source SO according to an embodiment of the invention. Theapparatus comprises: an illumination system (illuminator) IL configuredto condition a radiation beam B (e.g., EUV radiation); a supportstructure (e.g., a mask table) MT constructed to support a patterningdevice (e.g., a mask or a reticle) MA and connected to a firstpositioner PM configured to accurately position the patterning device; asubstrate table (e.g., a wafer table) WT constructed to allow holding ofa substrate (e.g., a resist-coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate; and aprojection system (e.g., a reflective projection system) PS configuredto project a pattern imparted to the radiation beam B by patterningdevice MA onto a target portion C (e.g., comprising one or more dies) ofthe substrate W.

The illumination system IL may include various types of opticalcomponents, such as refractive, reflective, magnetic, electromagnetic,electrostatic or other types of optical components, or any combinationthereof, for directing, shaping, or controlling radiation.

The support structure MT holds the patterning device MA in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem.

The term “patterning device” MA should be broadly interpreted asreferring to any device that can be used to impart a radiation beam witha pattern in its cross-section such as to create a pattern in a targetportion of the substrate. The pattern imparted to the radiation beam maycorrespond to a particular functional layer in a device being created inthe target portion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The projection system PS, like the illumination system, may includevarious types of optical components, such as refractive, reflective,magnetic, electromagnetic, electrostatic or other types of opticalcomponents, or any combination thereof, as appropriate for the exposureradiation being used, or for other factors such as the use of a vacuum.It may be desired to use a vacuum for EUV radiation since other gasesmay absorb too much radiation. A vacuum environment may therefore beprovided to the whole beam path with the aid of a vacuum wall and vacuumpumps.

As here depicted, the apparatus is of a reflective type (e.g., employinga reflective mask and/or reflective optics).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

Referring to FIG. 1, the illuminator IL receives an extreme ultra violetradiation beam from the radiation source SO. Methods to produce EUVradiation include, but are not necessarily limited to, converting a fuelmaterial into a plasma state that has at least one element, e.g., xenon,lithium or tin, with one or more emission lines in the EUV range. In onesuch method, often termed laser produced plasma (“LPP”) the requiredplasma can be produced by irradiating a fuel, such as a droplet, streamor cluster of material having the required line-emitting element, with alaser beam. The radiation source SO may include a laser, not shown inFIG. 1, for providing the laser beam exciting the fuel (the radiationsource and laser forming together a radiation system). The resultingplasma emits output radiation, e.g., EUV radiation, which is collectedusing a radiation collector, disposed in the radiation source. The laserand the radiation source may be separate entities, for example when aCO2 laser is used to provide the laser beam for fuel excitation.

In such cases, the laser is not considered to form part of thelithographic apparatus and the laser beam is passed from the laser tothe radiation source with the aid of a beam delivery system comprising,for example, suitable directing mirrors and/or a beam expander.

In an alternative method, often termed discharge produced plasma (“DPP”)the EUV emitting plasma is produced by using an electrical discharge tovaporise a fuel. The fuel may be an element such as xenon, lithium ortin which has one or more emission lines in the EUV range. Theelectrical discharge may be generated by a power supply which may formpart of the radiation source or may be a separate entity that isconnected via an electrical connection to the radiation source.

The illuminator IL may comprise an adjuster for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as facetted field and pupilmirror devices. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The EUV radiation beam B is incident on the patterning device (e.g.,mask) MA, which is held on the support structure (e.g., mask table) MT,and is patterned by the patterning device. After being reflected fromthe patterning device (e.g., mask) MA, the radiation beam B passesthrough the projection system PS, which focuses the beam onto a targetportion C of the substrate W. With the aid of the second positioner PWand position sensor PS2 (e.g., an interferometric device, linear encoderor capacitive sensor), the substrate table WT can be moved accurately,e.g., so as to position different target portions C in the path of theradiation beam B. Similarly, the first positioner PM and anotherposition sensor PS1 can be used to accurately position the patterningdevice (e.g., mask) MA with respect to the path of the radiation beam B.Patterning device (e.g., mask) MA and substrate W may be aligned usingmask alignment marks M1, M2 and substrate alignment marks P1, P2.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure (e.g., mask table) MT and thesubstrate table WT are kept essentially stationary, while an entirepattern imparted to the radiation beam is projected onto a targetportion C at one time (i.e., a single static exposure). The substratetable WT is then shifted in the X and/or Y direction so that a differenttarget portion C can be exposed.

2. In scan mode, the support structure (e.g., mask table) MT and thesubstrate table WT are scanned synchronously while a pattern imparted tothe radiation beam is projected onto a target portion C (i.e., a singledynamic exposure). The velocity and direction of the substrate table WTrelative to the support structure (e.g., mask table) MT may bedetermined by the (de-)magnification and image reversal characteristicsof the projection system PS.

3. In another mode, the support structure (e.g., mask table) MT is keptessentially stationary holding a programmable patterning device, and thesubstrate table WT is moved or scanned while a pattern imparted to theradiation beam is projected onto a target portion C. In this mode,generally a pulsed radiation source is employed and the programmablepatterning device is updated as required after each movement of thesubstrate table WT or in between successive radiation pulses during ascan. This mode of operation can be readily applied to masklesslithography that utilizes programmable patterning device, such as aprogrammable mirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIG. 2 shows the lithographic apparatus 100 in more detail, includingthe radiation source SO, the illumination system IL, and the projectionsystem PS. The radiation source SO is constructed and arranged such thata vacuum environment can be maintained in an enclosing structure 220 ofthe radiation source SO. An EUV radiation emitting plasma 210 may beformed by a laser or a discharge produced plasma source. EUV radiationmay be produced by a gas or vapor, for example Xe gas, Li vapor or Snvapor in which the very hot plasma 210 is created to emit radiation inthe EUV range of the electromagnetic spectrum. The very hot plasma 210is created by, for example, an electrical discharge or by a laser beamexcitation of fuel droplets, causing an at least partially ionizedplasma. Partial pressures of, for example, 10 Pa of Xe, Li, Sn vapor orany other suitable gas or vapor may be required for efficient generationof the radiation. In an embodiment, a plasma of excited tin (Sn) isprovided to produce EUV radiation.

The radiation emitted by the hot plasma 210 is passed from a sourcechamber 211 into a collector chamber 212 via an optional gas barrier orcontaminant trap 230 (in some cases also referred to as contaminantbarrier or foil trap) which is positioned in or behind an opening insource chamber 211. The contaminant trap 230 may include a channelstructure. Contamination trap 230 may also include a gas barrier or acombination of a gas barrier and a channel structure. The contaminanttrap or contaminant barrier 230 further indicated herein may at leastinclude a channel structure, as known in the art.

The collector chamber 212 may include a radiation collector CO which maybe a so-called grazing incidence collector. Radiation collector CO hasan upstream radiation collector side 251 and a downstream radiationcollector side 252. Radiation that traverses collector CO can bereflected off a grating spectral filter 240 to be focused in a virtualsource point IF. The virtual source point IF is commonly referred to asthe intermediate focus, and the radiation source is arranged such thatthe intermediate focus IF is located at or near an opening 221 in theenclosing structure 220. The virtual source point IF is an image of theradiation emitting plasma 210.

Subsequently the radiation traverses the illumination system IL, whichmay include a facetted field mirror device 222 and a facetted pupilmirror device 224 arranged to provide a desired angular distribution ofthe radiation beam 223, at the patterning device MA, as well as adesired uniformity of radiation intensity at the patterning device MA.Upon reflection of the beam of radiation 223 at the patterning deviceMA, held by the support structure MT, a patterned beam 226 is formed andthe patterned beam 226 is imaged by the projection system PS viareflective elements 228, 229 onto a substrate W held by the wafer stageor substrate table WT.

More elements than shown may generally be present in illumination opticsunit IL and projection system PS. The grating spectral filter 240 mayoptionally be present, depending upon the type of lithographicapparatus. Further, there may be more mirrors present than those shownin the Figures, for example there may be 1-6 additional reflectiveelements present in the projection system PS than shown in FIG. 2.

Collector optic CO, as illustrated in FIG. 2, is depicted as a nestedcollector optic with grazing incidence reflectors 253, 254 and 255, justas an example of a collector (or collector mirror). The grazingincidence reflectors 253, 254 and 255 are disposed axially symmetricaround an optical axis O and a collector optic CO of this type may beused in combination with a discharge produced plasma source, oftencalled a DPP source, however it may also be used in combination withlaser produced plasma where fuel droplets are excited for example withlaser beam energy into plasma state.

An alternative radiation source SO according to an embodiment of theinvention is shown schematically in FIGS. 3 and 4. The radiation source,which is shown viewed from above in FIG. 3 and viewed from one side inFIG. 4, is an LPP radiation source.

The radiation source SO comprises a fuel droplet emitter 10 which isconfigured to deliver fuel droplets to a plasma formation location 12. Alaser LA is arranged to deposit laser energy into the fuel droplets atthe plasma formation location 12, thereby forming a highly ionisedplasma 14 with electron temperatures of several 10's of eV. Theenergetic radiation generated during de-excitation and recombination ofthese ions is emitted from the plasma 14. This energetic radiationincludes EUV radiation. The radiation is collected by a near normalincidence collector optic CO and focused to an intermediate focus IF,from where the radiation passes into an illumination system IL of thelithographic apparatus (see FIGS. 1 and 2).

The radiation source SO further comprises a housing 16 within which acontrolled environment is provided, the plasma formation location 12 andcollector optic CO being located within the housing. Control of theenvironment may for example comprise providing a desired vacuum withinthe housing 16 and/or providing one or more desired gases at desiredpressures (the desired pressures may be significantly below atmosphericpressure and may thus be considered to be a vacuum). An opening 18 isprovided at one end of the housing 16, the position of the openingsubstantially corresponding with the position of the intermediate focusIF. An opening 20 (or window) is provided at an opposite end of thehousing in order to allow laser radiation to pass from the laser LA intothe housing. The opening 20 may be at any suitable location, for exampleon a side of the housing such that the laser beam forms an angle with anoptical axis O of the radiation source SO. Other openings may also beprovided for further lasers needed to produce the plasma (such asanother main-pulse laser for exciting the fuel and/or a pre-pulse laserfor evaporating the fuel, as known in the art).

The radiation source SO includes a bar 60 which extends substantiallyhorizontally across the interior of the housing 16 of the source andintersects with the optical axis O. The bar 60 acts to block laserradiation emitted by the laser LA, thereby preventing that laserradiation from being directly incident upon optical surfaces in theilluminator IL and projection system PS of the lithographic apparatus(see FIGS. 1 and 2). The bar 60, which may be referred to as theobscuration bar 60, is described in more detail further below.

In the following description the fuel which is emitted from the fueldroplet emitter 10 is tin. However, other fuels may be used, for examplexenon or lithium.

The fuel droplet emitter 10 may be configured to operate continuously.That is, the fuel droplet emitter 10 may be heated to a temperaturewhich is above the melting temperature of the fuel (e.g., tin), and mayremain at that temperature and continuously emit droplets of fuel untilreplacement of the fuel droplet emitter is required. Consequently, theremay be periods of time during which laser radiation is not being emittedby the laser LA but droplets of fuel continue to be emitted from thefuel droplet emitter 10. A fuel catcher 22 is provided on an oppositeside of the source SO from the fuel droplet emitter 10. The fuel catcher22 may comprise a container such as a tube which is configured toreceive and retain fuel droplets which have been emitted by the fueldroplet emitter 10 and which have not been vaporised by the laserradiation.

A plurality of vanes 24 a-d extend from the housing 16. The vanes mayfor example be made from molybdenum, or from stainless steel providedwith a galvanic tin plating. The vanes may be provided with a surfacewhich retains incident liquid tin (i.e. a good wetting surface for thefuel material). A tin surface (e.g., obtained using galvanic tinplating) provides affinity with liquid tin such that the surface willretain the liquid tin. The vanes may be provided with a surface whichdoes not react significantly with liquid tin at working temperatures ofthe radiation source. For example, in the case of a stainless steelvane, the vane may be held at a temperature above 250° C. (at whichtemperature the tin may remain in a liquid state), and may be held at atemperature which is below 400° C. In the case of a molybdenum vane, thevane may be held at a temperature above 250° C., and may be held at atemperature which is below around 1100° C. (which may be the boilingpoint of tin at radiation source pressure). The vanes may be heatedusing any suitable heating apparatus (not illustrated).

Although only two vanes 24 a,b are shown in FIG. 3 and two vanes 24 c,dare shown in FIG. 4, more than four vanes may be provided. For example,vanes may be distributed around an inner wall of the housing 16. Forexample, twenty or more vanes, forty or more vanes, or sixty or morevanes may be distributed around an inner wall of the housing 16. Thevanes 24 a-d may extend generally radially inwards from the housing 16.The vanes 24 a-d may subtend one or more angles relative to the radialdirection. The vanes 24 a-d may be flat. Alternatively, the vanes mayinclude some curvature. Different vanes 24 a-d may have different shapesand/or orientations relative to the radial direction, for example totake into account the angled upward orientation of the housing 16 (asshown in FIG. 4). A gutter 27 is located at one end of the vanes 24 a-d.The gutter 27 may be connected to a drain (not shown). The vanes 24 a-dmay include support structures 29 which may be located at an oppositeend of the vanes from the gutter 27.

Also extending from the housing 16 are reflective structures 26. Thereflective structures 26 are located in the vicinity of the intermediatefocus IF, and are configured to reflect EUV radiation. Some radiationwhich is incident upon the collector optic CO may be generally focusedtowards the intermediate focus IF, but may not be sufficiently stronglyfocused to pass through the intermediate focus IF. This radiation willbe incident upon the reflective structures 26. The reflective structures26 may for example be arranged to reflect this radiation back into thehousing 16, thereby reducing the likelihood of radiation which is notstrongly focused passing through the opening 18 and into theillumination system IL (see FIGS. 1 and 2). The reflective structures 26may for example comprise a series of vanes which extend substantiallycircumferentially around the housing 16 in the vicinity of theintermediate focus IF. The reflective structures 26 may have a generallyfrustoconical shape.

As described further above, a tin droplet which is delivered to theplasma formation location 12 is vaporised by the laser beam emitted fromthe laser LA to generate a radiation emitting plasma 14. However,vaporisation of the tin droplet may be incomplete, and as a resultresidual droplets of tin may remain after the plasma 14 has been formed.It is desirable to reduce the likelihood of these residual tin dropletsbeing incident upon the collector optic CO or passing through theopening 18 into the illumination system IL. The vanes 24 a-d act astraps which receive the residual tin droplets, being thus an example ofa fuel debris receiving surface.

The vanes 24 a-d (or other vanes of the radiation source SO) may beheated by a heating apparatus. The vanes 24 a-d may be heated to atemperature which is above the melting temperature of tin (e.g., toabove around 230° C., e.g. to above around 250° C.). As a result,residual tin droplets which are incident on the vanes 24 a-d remain inliquid form and may flow along or across the vanes. The gutter 27receives the liquid tin from the vanes 24 a-d and directs the liquid tinto a drain (not shown).

The temperature of the vanes (e.g., vanes 24 a-d) may be below theevaporation temperature of tin (i.e., the temperature at which asignificant degree of evaporation of tin will occur). This depends uponthe pressure in the radiation source, and may for example be in therange from 1100° C. to 1600° C. It may be preferable not to cause thetin to evaporate, because the evaporated tin could travel to andcondense upon other surfaces in the radiation source SO (or elsewhere),such as optical surfaces.

As will be appreciated from FIG. 4, because the radiation source SO hasan angled upward orientation, some of the vanes 24 c are located overthe collector optic CO. Grooves are provided in the surfaces of thevanes 24, the grooves being schematically represented by lines 28. Thegrooves 28 have orientations which are arranged to direct the flow ofliquid tin under the influence of gravity towards the gutter 27. Thegrooves 28 thus reduce the likelihood of liquid tin dripping from a vane24 c onto the collector optic CO. This is advantageous becauseaccumulation of tin on the collector optic CO may modify thereflectivity of the collector optic CO in an undesirable manner.

The vanes 24 may thus be considered to be examples of immobile fueldebris receiving surfaces. Grooves may be provided in other immobilefuel debris receiving surfaces in the radiation source, such as on thewalls, between vanes, on an obscuration bar surface, etc.

Droplets of liquid tin which are located close to a lowermost edge of avane are the most likely drip off the vane. Consequently, it may beadvantageous to provide grooves at, or adjacent to, a lowermost edge ofa vane. It may also be advantageous to provide grooves at location(s) ona vane which are most likely to receive tin debris during operation ofthe radiation source.

The grooves may have a cross-sectional shape which provides capillaryaction and/or wicking action, thereby promoting the flow of liquid fuelinto the grooves (and inhibiting the flow of liquid fuel across thegrooves). The grooves may for example have a v-shaped cross-section.Alternatively, the grooves may have a rectangular cross-section, e.g.,having two corners which each subtend an angle of around 90°. In thiscase, the two corners may provide the force which gives rise tocapillary action and/or wicking action. The grooves may have any othersuitable shape, which may for example provide capillary action and/orwicking action. For example, a corner may extend longitudinally alongthe groove, the corner giving rise to capillary action and/or wickingaction.

A set of grooves 28 on a vane 24 may comprise grooves which extendsubstantially parallel to each other. The substantially parallel groovesmay be straight or curved, or a combination of both. The grooves mayalso be interconnected.

Although only three grooves 28 are shown on each vane 24, this is merelya schematic example. Three or more grooves may be provided on each vane24. For example, ten or more, or twenty or more grooves may be providedon each vane 24. The spacing between adjacent grooves may for example be6 mm or less. 6 mm is twice the capillary length for liquid tin(capillary length is intended to mean the maximum distance from thegroove at which capillary action can draw tin into the groove). Thus, iftwo grooves are separated by 6 mm, the entire surface between thosegrooves may be within the capillary length of one of the grooves. Thespacing between adjacent grooves may for example be around 3 mm (i.e.,the capillary length for liquid tin). The spacing between adjacentgrooves may for example be 1 mm or more. A spacing of around 1 mm couldeffectively create a fully grooved surface.

The grooves may for example have a depth of at least 0.1 mm. The groovesmay for example have a depth of up to 0.2 mm, a depth of up to 0.5 mm,or a depth of up to 2 mm. The grooves may for example have a depth of0.5 mm.

If a groove has a v-shaped cross-section then the width of the groovewill be determined by its depth and its opening angle. A groove (e.g., av-shaped groove) may for example have a width of at least 0.1 mm. Agroove (e.g., a v-shaped groove) may for example have a width of up to0.2 mm, a width of up to 0.5 mm, or a width of up to 2 mm.

If the groove has a rectangular cross-section then the width of thegroove will be independent of its depth. This allows the groove to beprovided with a greater width in order to increase draining capacityprovided by the groove. A rectangular groove may for example have awidth of up to 0.2 mm, a width of up to 0.5 mm, a width of up to 2 mm,or a width of up to 10 mm. A rectangular groove may for example have awidth of at least 0.1 mm.

FIG. 5 shows schematically in cross-section a v-shaped groove which mayfor example be formed in a vane surface or another surface inside theradiation source SO in accordance with an embodiment of the invention.The v-shaped groove 28 is shown relative to an x-direction, which mayfor example be parallel to a surface of the vane. A normal to thex-direction is also shown. The v-shaped groove 28 has an opening angleβ. Each side of the v-shaped groove 28 subtends an angle α relative tothe x-direction (referred to hereafter as the elevation angle) such that2α+β=180°. The v-shaped groove has a depth h0.

Also shown in FIG. 5 is liquid tin 30 (although also other liquid fuelscan be envisaged). The liquid tin 30 has a meniscus 32 which has aradius of curvature R. The meniscus subtends an angle θ at the locationwhere the meniscus contacts the surface of the v-shaped groove 28.

Capillary action of a droplet of liquid in a groove may cause a wettingfront of the droplet to advance (i.e., for the droplet to spread out inthe groove). This advance of the droplet wetting front may becharacterised by the following equation:D=(σh ₀/μ)^(1/2) K(θ₀,α)  (1)

where D is a transport coefficient which characterises the rate ofadvance of the wetting front of the droplet, α is the surface tension ofthe liquid, h0 is the depth of the groove, μ is the dynamic viscosity ofthe liquid, and K is a geometrical function of the elevation angle α andthe angle θ₀ formed between the meniscus and the groove before the fluidstarts to move. As will be appreciated from the above equation, thetransport coefficient D is proportional to K.

FIG. 6 is a graph which shows how K varies as a function of theelevation angle α (a being expressed in radians). As may be seen fromFIG. 6, K passes through a maximum at an elevation angle α of around 70°(around 0.77 on the horizontal scale of FIG. 6). This corresponds to anopening angle θ of the groove of around 40°. Thus, capillary action of atin droplet in the v-shaped groove may be at a maximum for a grooveopening angle θ of around 40°. As will be appreciated from FIG. 6, otheropening angles β may also give rise to significant capillary action. Thegroove opening angle β may for example be in the range 30°-50°, and mayfor example be in the range 20°-60°.

More information regarding the effect of the shape of a groove uponliquid flowing into the groove may be found in L. A. Romero et al, J.Fluid Mech. (1996), vol. 322, pp 109-129, which is herein incorporatedby reference.

As explained above, the capillary action provided by a v-shaped groove28 may cause a tin droplet to spread out along the groove. In addition,the v-shaped groove 28 may draw liquid tin into the v-shaped groove froma surrounding area. This may occur for example if part of a liquid tindroplet overlaps with the groove and part of the liquid tin droplet doesnot overlap with the groove. A wicking action arising from forcegenerated by the liquid tin in the groove will draw the droplet into thegroove. This drawing of a liquid tin droplet into the groove via wickingaction is advantageous because it prevents or inhibits the flow ofliquid tin across the groove.

Referring to FIG. 4, if liquid tin is received on a vane 24 c which islocated above the collector optic CO, gravity will urge that liquiddroplet to flow downwards towards a bottom edge of the vane. In theabsence of the grooves 28 there would be a risk that the liquid tinwould drip from the vane 24 c and be incident upon the collector opticCO. However, when the liquid tin flows over a groove 28 (e.g., av-shaped groove), the wicking action provided by the groove will drawthe liquid tin into the groove, and will inhibit the liquid tin fromcontinuing to flow downwards and across the vane 24 c. The groove 28directs the flow of liquid tin under the influence of gravity towardsthe gutter 27. The vane 24 c is thus configured to receive liquid tinand direct that liquid tin via grooves 28 to the gutter 27. In additionto gravity, capillary action in the grooves 28 may also drive flow ofliquid tin towards the gutter 27.

In the case of a groove having a rectangular cross-section, flowresistance due to a side wall of the groove may prevent overrunning offluid in a direction which runs across the groove (i.e., the flowresistance may keep the fluid within the groove). The fluid will followthe direction of the groove, being directed along the groove under theinfluence of gravity. Corners running along the rectangular groove mayalso give rise to capillary action and/or wicking action. In general, acorner (i.e. an edge) which runs along a groove may give rise tocapillary and/or wicking action.

The grooves 28 in the right hand vane 24 d of FIG. 4 also promote theflow of liquid tin, such that the liquid tin flows into the gutter 27.In this instance the vane does not extend above the collector optic CO,and has an orientation which is closer to vertical than the vane 24 creferred to above. The grooves 28 may therefore not be required toprevent dripping of liquid tin onto the collector. Nevertheless, thegrooves 28 may direct the flow of liquid tin under the influence ofgravity towards the gutter 27.

In general, the grooves 28 are oriented such that they direct the flowof liquid tin under the influence of gravity towards the gutter 27. Thegrooves 28 may be configured to cause the liquid tin to flow morerapidly than would be the case if the grooves were not present. This mayallow the vanes 24 to receive liquid tin at a greater flow rate thanwould otherwise be possible, without that liquid tin filling the vanesand giving rise to dripping of excess liquid tin onto the collectoroptic CO (or onto other surfaces).

The effect of gravity on the flow of liquid tin received by the vanes 24is considered below. FIG. 7 schematically shows a v-shaped groove 28which is tilted relative to the horizontal such that gravity promotesthe flow of liquid tin 30 along the v-shaped groove, as indicated byarrow 34. A downwardly pointing arrow g schematically representsgravitational force. The angle φ of the groove 28 relative to thehorizontal is also represented schematically (this angle is hereafterreferred to as the tilt angle φ, which can also be represented as angle(90°−φ) relative to the vertical direction of the gravity force). In thesituation shown in FIG. 7 the liquid tin 30 may flow through thev-shaped groove 28 due to a pressure gradient p7 which is induced bygravity. The volumetric flux q of the liquid tin (i.e., rate of flow ofliquid tin) is given by:

$\begin{matrix}{q = {{- \frac{h^{4}}{\mu}}{\Gamma\left( {\vartheta_{0},\alpha} \right)}\rho\; g\mspace{11mu}\sin\mspace{11mu}\varphi}} & (2)\end{matrix}$where h is the depth of the groove, μ is dynamic viscosity of the liquidtin, Γ is a geometrical function of the elevation angle α and the angleθ₀ formed between the meniscus and the groove before the liquid tinstarts to move,

is the density of the liquid tin, g is acceleration due to gravity, andφ is the tilt angle of the groove. Additional information regarding theflow of liquid along a groove due to gravity may be found in the paperby Romero et al referred to further above.

FIG. 8 is a graph which shows how the geometrical function Γ varies as afunction of the elevation angle α of the groove. As may be seen, thegeometrical function Γ, and hence the volumetric flux (which isproportionate to this function), is greatest for a small elevation angleα and reduces as the elevation angle α increases.

FIG. 8 taken in combination with FIG. 6 indicates that there may be atrade-off between the volumetric flux of liquid tin provided by a grooveand the capillary action provided by the groove (due to the effect ofthe elevation angle α). An elevation angle α of around 70° will causeliquid to be strongly drawn into a groove, and for a liquid droplet tospread out quickly along that groove. However, an elevation angle α ofaround 70° may significantly reduce the flow rate of liquid along thegroove, compared for example with an elevation angle α of 20° (asdemonstrated by FIG. 8).

As may be seen from equation (2), the volumetric flux of the liquidscales to the 4th power of the depth of the groove. Therefore, the depthof the groove has a stronger effect upon the volumetric flux than theelevation angle α. This may be seen from FIG. 9 which is a graph whichshows how the volumetric flux varies as a function of the tilt angle φof the groove (see FIG. 7) for grooves with two different depths. Bothgrooves have an elevation angle α of 70°. As may be seen, the volumetricflux increases as the tilt angle φ increases. The graph in FIG. 9 showsthe volumetric flux for a groove having a depth of 0.1 mm and a groovehaving a depth of 0.2 mm. As may be seen, the volumetric flux for thegroove having a depth of 0.2 mm is very much greater than the volumetricflux for the groove having a depth of 0.1 mm.

Since the effect of the depth of the groove on the volumetric flux isstronger than the effect of the elevation angle α, grooves formed in thevanes 24 may for example be formed with a elevation angle α which givesrise to a desired capillary effect (e.g., an angle of around 70°), andthe depth of the groove may then be selected in order to provide adesired volumetric flux.

In an alternative approach, for a groove 28 which requires strongcapillary action to retain liquid tin, the elevation angle α may begiven priority, whereas for a groove which does not require strongcapillary action the flow rate may be given priority. Referring to FIG.4, the grooves 28 which are located on the vane 24 c above the collectoroptic CO may be considered to be grooves which require strong capillaryaction. The grooves 28 which are located on the vane 24 d to the righthand side of the source SO may be considered to be grooves which do notrequire strong capillary action (because the orientation of the groovesis close to vertical).

If the grooves 28 were not present on the vanes 24, then an imperfectionsuch as a scratch could inhibit or prevent passage of tin in a desireddirection on a vane. For example, a vertical scratch on the vane 24 cwhich is above the collector optic CO in FIG. 4 could inhibit movementof liquid tin towards the gutter 27, and could promote downward movementof the liquid tin (and subsequent dripping onto the collector optic CO).This may be prevented by the grooves 28, provided that the grooves aredeeper than any scratches (which may be expected to be the case).

In the absence of the grooves 28, droplets of tin (or other fuel) with adiameter of between 10 microns and 10 mm could accumulate on the vanes24. When the grooves are present in the vanes 24, the maximum size oftin droplets formed on the vanes 24 may be limited by the grooves, dueto the wicking action of grooves drawing tin into the grooves (describedfurther above).

In the this description the grooves 28 have been described as beingprovided on the vanes 24. This may include providing grooves on thesupport structures 29 which form part of the vanes.

Grooves may be provided at other locations within the radiation sourceSO. For example, grooves may be provided in a wall of the radiationsource housing 16. Grooves may be provided between vanes 24 which extendfrom the housing. Grooves may be provided on any immobile fuel debrisreceiving surface within the radiation source SO. In this context, theterm “fuel debris receiving surface” may be interpreted as meaning asurface which receives fuel debris (e.g., liquid tin droplets) duringoperation of the radiation source. The term “immobile” may beinterpreted as meaning that the surface does not move during operationof the radiation source.

Grooves may be provided on the reflective structures 26. The reflectivestructures 26 may be considered to be vanes. The grooves may for exampleextend substantially circumferentially around the reflective structures26, or may extend radially, or may have a compound partially radial andpartially circumferential shape. The grooves may have any other suitableform. The grooves may have orientations which are arranged to direct theflow of liquid tin, under the influence of gravity, to a drain (notshown).

In an embodiment, the tin catcher 22 may be provided with a series ofvanes, for example as shown schematically in FIG. 10. The vanes 36 mayextend inwardly from walls of the tin catcher 22, and may be configuredto reduce the likelihood of liquid tin which enters the tin catchersplashing back out of the tin catcher. The vanes 36 may be provided withgrooves which have orientations which are arranged to direct the flow ofliquid tin in a desired direction (e.g., towards the bottom of the tincatcher). In an embodiment, the vanes 36 may comprise one or morehelixes which extend around an inner circumference of the tin catcher22. One or more grooves may extend along substantially all of a givenhelical vane 36. The grooves may have any suitable form.

Grooves may be provided on vanes at other locations in the radiationsource SO.

The grooves provided on the reflective structures 26 or on vanes 36 ofthe tin catcher 22 (or on other surfaces) may have one or more of theproperties referred to further above in relation to the vanes 24.

The grooves may be inclined relative to the horizontal, in order that acomponent of force due to gravity causes liquid tin to flow along thegrooves.

The grooves may be considered to direct the flow of liquid tin under theinfluence of gravity.

In this description, the term “elevation angle” is not intended to implythat the vane surface in which the groove is formed must be horizontal.

FIG. 11 shows schematically viewed from one side in partialcross-section a substantially conical portion of a radiation sourcehousing 40. Other parts of the radiation source are not shown, but maycorrespond with parts of the radiation source SO shown in FIG. 4. Aplurality of vanes 42 extend inwardly from the housing 40. The vanes 42may be distributed around the housing 40. Although seven vanes are shownin FIG. 11, this is merely a schematic illustration and any suitablenumber of vanes may be provided. For example, twenty or more vanes,forty or more vanes or sixty or more vanes may extend inwardly from thehousing 40. A gutter 44 is located at a bottom end of the vanes 42. Atin collector 46 is connected to the gutter 44.

A liquid tin inlet 48 is located at or adjacent to upper ends of thevanes 42. The liquid tin inlet 48 delivers a flow of liquid fuel ontothe vanes 42. Liquid tin delivered via the liquid tin inlet 48 flowsalong and/or between the vanes 42 and is received in the gutter 44, fromwhere it flows into the tin collector 46 (i.e. a tin catcher such ascatcher 22 in FIG. 3).

The portion of FIG. 11 which is enclosed by a dotted elliptical line isshown schematically in more detail in cross-section in FIG. 12. Twovanes 42 are shown in FIG. 12. Part of the liquid tin inlet 48 is alsoshown. The liquid tin inlet 48 comprises openings 50 between the vanes42 through which a flow of liquid tin is delivered. The openings 50 areconnected to a conduit 52 which acts as a supply of liquid tin to theopenings 50. The conduit 52 may be connected to a reservoir or someother source of liquid tin.

A heater, in this case heating elements 54, is located behind theconduit 52. The heating elements heat the conduit 52 and the vanes 42such that they are both above the melting temperature of tin, therebyensuring that the tin remains in liquid form. The heating elements 54may for example heat the conduit 52 and vanes 42 to a temperaturebetween 250° C. and 350° C. Part of the housing wall 40 is also shown inFIG. 12.

In use, liquid tin is delivered via the conduit 52 into the openings 50during operation of the EUV lithographic apparatus. The liquid tin isrepresented by grey shading 55 in FIG. 12. The tin is maintained inliquid form by the heating elements 54. On flowing through the openings50 the liquid tin 55 is delivered onto tin debris-receiving surfaces ofthe vanes 42 (i.e., outer surfaces of the vanes). A coating of liquidtin 55 is thereby provided on the vanes 42. The coating of liquid tin isnot stationary, but rather flows downwards along the vanes. This isindicated schematically by arrows 56 in FIG. 11. This downward flow ofthe coating of liquid tin is due at least in part to gravity. When theliquid tin reaches the bottom of the vanes 42 it enters the gutter 44and then flows into the tin collector 46. Liquid tin may be returnedfrom the collector to the liquid tin inlet 48 and thereby reintroducedonto the tin debris receiving surfaces of the vanes 42. The liquid tininlet 48 may be configured to provide a flow of liquid tin to the tindebris receiving surface of the vanes 42. The flow of liquid tin may forexample be a continuous supply of liquid tin. The flow of liquid tin mayensure that the tin debris receiving surface of the vanes 42 iscontinuously covered with a coating of liquid tin. The coating of liquidtin may ensure that the tin debris receiving surface of the vanes 42 isa good wetting surface.

The term “wetting” refers to the ability of a liquid to maintain contactwith a solid surface, arising from intermolecular interactions when theliquid and solid surface are brought together. Adhesive forces between aliquid and solid will tend to cause a liquid drop to spread across thesolid surface. Cohesive forces within the liquid will tend to cause theliquid drop to take a spherical form and avoid contact with the solidsurface. In an EUV lithographic apparatus, as has been explained furtherabove, it may be desirable to retain liquid tin debris on adebris-receiving surface, and conversely it may be undesirable for theliquid tin debris to drip off the debris-receiving surface. Thus, it maybe desirable to ensure that the debris-receiving surface is a goodwetting surface.

The vanes 42 may be made from stainless steel. Stainless steel may havepoor wetting properties. Therefore, tin debris receiving surfaces of thevanes 42 may be pre-coated with a layer of tin (tin is a good wettingsurface). However, it may be the case that during operation of the EUVlithographic apparatus the layer of tin provided on the stainless steelvanes will drain away, leading to exposed areas of stainless steelsurface. Such exposed areas of stainless steel surface are undesirablebecause, due to the poor wetting nature of stainless steel, they mayinterfere with the flow of liquid tin debris to the gutter 44. Forexample, an exposed stainless steel area may inhibit flow of liquid tin,causing liquid tin to accumulate above the exposed stainless steel area.This accumulation of tin could become sufficiently large that it dripsoff the vane due to gravity. This is undesirable because the drip couldfor example be incident upon the collector optic CO (see FIG. 4) of theradiation source SO, thereby compromising the effectiveness of thecollector.

The embodiment of the invention shown in FIGS. 11 and 12 overcomes theabove problem by providing a flow of liquid tin through liquid tininlets 48, such that a coating of liquid tin is maintained on the vanes42. The liquid tin prevents or inhibits stainless steel of the vanesfrom becoming exposed. Thus, a good wetting surface (i.e., liquid tin)is present on the vanes. The good wetting surface promotes thecontrolled flow of liquid tin debris as a thin coating, such that theliquid tin debris flows into the gutter 44 (not shown). Liquid tin maybe provided to other debris receiving surfaces, as described furtherbelow.

The liquid tin inlet 48 may comprise openings 50 located between vanes42, as schematically shown in FIG. 12. Although only one opening 50 isshown between each vane 42, a plurality of openings may be providedbetween the vanes. In an embodiment, the liquid tin inlet 48 maycomprise one or more arrays of micro-pores. The micro-pores may forexample have diameters of the order of 100 μm. The liquid tin inlet 48may comprise a series of openings distributed around the circumferenceof the housing 40. The openings 50 may for example be located betweenthe vanes 42 or adjacent to the vanes. The openings 50 may for examplebe located at or adjacent to upper ends of the vanes 42. The openingsmay be provided at any other suitable location. The liquid tin inlet 48may have any suitable form.

The liquid tin inlet 48 may be connected to a fuel debris receivingsurface, for example opening directly onto the fuel debris receivingsurface or opening onto a surface which is connected to the fuel debrisreceiving surface (e.g., such that the liquid tin flows over thatsurface and onto the fuel debris receiving surface). The liquid tininlet 48 may comprise one or more openings which are spaced apart fromthe fuel debris receiving surface, for example such that liquid tinfalls under the influence of gravity onto the fuel debris receivingsurface. The liquid tin which falls onto the fuel debris receivingsurface, which may be in the form of a continuous stream, may beconsidered to be an example of a flow of liquid fuel onto the debrisreceiving surface.

The vanes 42 may extend radially inwards towards an optical axis of theradiation source. The vanes may have a concave shape. The vanes may havesharp tips. The vanes may have a shape which is arranged to minimisesurface area that is perpendicular to the radial direction, therebyminimising the likelihood of tin debris bouncing directly back from avane into the radiation source. The vanes may be straight, as shown inFIG. 11, or may be curved.

The vanes 42 may be provided with a curved surface which leads to anon-uniform capillary pressure that draws the liquid tin 55 away fromtips of the vanes. This may be advantageous because it prevents thebuild-up of large amounts of liquid tin 55 at tips of the vanes 42,whilst maintaining a coating of liquid tin on the vanes and therebymaintaining a good wetting surface.

FIG. 13 is a plot which shows an example of a vane profile which couldbe used. The vane 42 extends downwardly in FIG. 13, the interior of theradiation source (not shown) being below the line of the plot. Theopenings 50 are not shown in the plot (they may for example be upstreamof the vane profile shown in the plot). The curvature of the vane 42 isat a maximum at the tip of the vane, and decreases as the distance fromthe tip of the vane increases. As a result of this curvature, a gradientin the capillary pressure of a thin layer (which may be considered to bea coating) of liquid tin of around 0.7 mbar/mm will be generated. Thisis around the same magnitude as the pressure gradient which is due tothe effect of gravity. Thus, the shape of the vane 42 shown in the plotof FIG. 13 will inhibit or prevent gravity causing the accumulation ofliquid tin at the tip of the vane. Instead, a coating of liquid tin 55will be provided in the vicinity of the tip of the vane, with theremainder of the liquid tin being drawn into a space 45 between adjacentvanes 42. The coating of liquid tin 55 may for example have a thicknessof around 100 microns or less (unwanted dripping from the vane tip mayoccur for coating thicknesses which are well in excess of this value).

As may be seen from FIG. 13, there is a corner 43 on either side of thevane 42 at the position where that vane meets a neighbouring vane.Spaces 45 between the vanes 42 may be considered to be grooves 45. Inthe vicinity of the corners 43, the profile of the vane 42 issubstantially linear. The corners 43 of the grooves 45, together withthe shape of the profile in the vicinity of the corners 43 thus formv-shaped grooves. The v-shape of the grooves 45 generates capillarypressure in the liquid tin which retains liquid tin in the grooves bydrawing the liquid tin into the grooves. The capillary pressure alsopromotes spreading of liquid tin along the grooves 45, leading to a filllevel 58 of liquid tin being established (see FIG. 12).

The vanes 42 of the embodiment shown in FIGS. 12 and 13 may be separatedby 6 mm or less. As noted further above, 6 mm is twice the capillarylength for liquid tin (for other fuels the capillary length may bedifferent). For a given separation between vanes 42, the heights of thevanes may be selected such that a gradient of curvature may beestablished in the vicinity of the tips of the vanes which prevents orinhibits the accumulation of liquid tin at the tips of the vanes. Thus,for vanes 42 which are separated by around 6 mm, the vanes may forexample have a height of around 3 mm. This may provide sufficient heightto accommodate a non-uniform curvature which generates capillarypressure that draws liquid tin away from tips of the vanes 42. The vanes42 may have any suitable height. Selection of the height of the vanes 42may take into account the extent to which the vanes reduce or preventback-scattering of liquid tin that has been incident upon the vanes 42(a greater height may be more effective in reducing or preventingback-scattering). The height of the vanes 42 may be of the order of thecapillary length of liquid tin.

As noted above, the separation between vanes 42 may be less than 6 mm.Small separations between vanes may be more difficult to manufacture,and manufacturability may thus limit the extent to which the separationbetween the vanes 42 can be reduced. In addition, smaller separationsbetween vanes may reduce the gradient of curvature which can be providedon the vanes (for a given vane height).

The grooves 45 between the vanes 42 have orientations which direct theflow of liquid tin under the influence of gravity towards the gutter 44.The grooves 45 may trap incident liquid tin debris.

Some of the coating of liquid tin 55 in the vicinity of the tip of thevane 42 may be provided by liquid tin provided from the liquid tin inlet48, and some of the coating of liquid tin may be provided by liquid tindebris. When liquid tin debris is incident upon the vane 42, excessliquid tin is automatically drawn towards the base of the vane bycapillary pressure. The excess liquid tin will then join liquid tinwhich is flowing along the base of the vane 42, and will be guided tothe gutter 44.

Referring again to FIG. 12, the curvature of the vanes 42 may be suchthat liquid tin 55 will tend to accumulate at a base of the vanes. Theliquid tin 55 may fill an area at the base of the vanes to a level 58which is determined, at least in part, by the rate at which liquid tinis delivered by the liquid tin inlet 48. The rate at which the liquidtin is supplied by the liquid tin inlet 48 may be selected such toprovide a desired fill level 58. For example, liquid tin 55 may bedelivered via the liquid tin inlet 48 at a rate which provides a filllevel of 1 mm or less (or some other value). Delivery of too much liquidtin 55 via the liquid tin inlet 48 may be undesirable because it couldlead to a fill level 58 which is undesirably high, and thus lead forexample to dripping of tin from the vanes 42. The liquid tin 55 mayautomatically distribute to a fill level 58 along the direction of flowof the liquid tin, due to capillary pressure effects generated by thegroove defined between the vanes 42.

In an alternative embodiment (not illustrated) liquid tin may besupplied via a liquid tin inlet to a radiation source wall which is notprovided with vanes, or to some other tin debris receiving surface (suchas exemplified above). Where this is the case, the supply of liquid tinwill maintain the wetting nature of the debris receiving surface in themanner described above, thereby inhibiting the accumulation of tin onthe surface and possible subsequent dripping of the tin from thesurface.

In an embodiment, a liquid tin inlet may be arranged to supply liquidtin to a surface such as a wall which does not have vanes or otherstructures. Where this is the case, the wall may be arranged such thatparts of the wall which receive liquid tin do not include any horizontalportions. The wall may be arranged such that parts of the wall whichreceive liquid tin subtend an angle of 20° or more relative to thehorizontal, and may be arranged such that parts of the wall whichreceive liquid tin subtend an angle of 30° or more relative to thehorizontal (i.e. subtend an angle of 60° or less relative to thevertical direction of the gravitational force). This may prevent orreduce dripping of liquid tin from the wall. The rate of supply ofliquid tin from the liquid tin inlet may be selected to keep thethickness of the coating of liquid tin on the wall at around 100 micronsor less.

Liquid tin may be supplied via a liquid tin inlet for example toreflective structures which are located in the vicinity of theintermediate focus IF (e.g., reflective structures 26 described furtherabove in connection with FIGS. 3 and 4). The reflective structures maybe fuel debris receiving surfaces.

Liquid tin may be supplied via a liquid tin inlet for example to anouter surface of the fuel droplet emitter 10 (see FIG. 3). The outersurface of the fuel droplet emitter (e.g., a fuel droplet emittingnozzle) may be a fuel debris receiving surface.

Liquid tin may be supplied via a liquid tin inlet for example to vanes24 provided with grooves 28 (described further above in connection withFIGS. 3 and 4). The vanes 24 may be fuel debris receiving surfaces.

In an embodiment, the one or more liquid tin inlets referred to abovemay be used to deliver a liquid other than liquid tin. The liquid tininlets may thus be considered to be liquid inlets generally. The liquidinlets may be used to deliver any suitable liquid. For example,Galinstan may be introduced through the liquid inlets. Galinstan isavailable from Geratherm Medical AG of Germany, and comprises gallium,indium and tin. The Galinstan preferably has such a concentration ofcomponents that the tin debris dissolved in it will be still in lowproportion, to avoid solidifying. These components may for example beprovided in the following proportions: around 68% gallium, around 22%indium and around 10% tin.

Galinstan has a melting point of −19° C., and thus is in liquid form atroom temperature (e.g. around 20° C.). Consequently, Galinstan may bearranged to flow down surfaces of the housing 40 when the housing is atroom temperature. Galinstan is a good wetting material, and thus mayform a coating on a surface over which it is flowing. Tin debris whichis incident upon the Galinstan may be captured by the Galinstan and flowdown surfaces of the housing 40 (e.g. in the manner described above). Inthis way, the tin debris is captured and delivered to the tin collector46 (along with the Galinstan). This capture of the tin debris isachieved without having to heat the housing 40 to a temperature abovethe melting temperature of tin (e.g. to above around 200° C.).

Not heating the housing 40 is advantageous for a variety of reasons. Forexample, if maintenance of a heated housing is needed there is a timedelay incurred waiting for the housing to cool down before it can behandled. This is avoided if the housing is unheated. Similarly,following maintenance of a heated housing a time delay is incurredwaiting for the housing to heat up to an operational temperature. Again,this is avoided if the housing is unheated. A further advantage is thatheaters and associated control electronics are not required, therebysimplifying the construction of the housing. A further advantage is thatoutgassing, which tends to increase with temperature, is reduced in thehousing.

Cooling of the housing 40 using for example water or other liquid may beused to dissipate heat generated as a by-product of EUV radiationgeneration. The cooling may for example be used to maintain at leastpart of the housing below 350° C., such as at room temperature (or anyother suitable temperature).

The Galinstan may be supplied periodically or continuously. TheGalinstan may be provided at a sufficient rate that tin which isincident upon the Galinstan does not significantly alter properties ofthe Galinstan. Tin which is incident upon the Galinstan and dissolvesinto the Galinstan could change the constitution of the Galinstan suchthat it is no longer liquid at room temperature. This may be avoided byproviding Galinstan at a sufficiently high rate, taking into account therate at which tin debris is incident upon surfaces of the housing 40.This may for example be determined experimentally,

In an embodiment, the vanes 42 may be formed from a porous metal, andthe Galinstan may be supplied from within the vanes. For example thehousing may be configured such that Galinstan travels through the vanesand passes out of front surfaces of the vanes. The Galinstan may forexample be delivered into the vanes 42 from a rear surface of the vanes.An advantage of delivering Galinstan through a porous metal is that thiswill automatically deliver more Galinstan to locations that receive themost tin debris. This is because a location which receives a lot of tindebris will have more Galinstan drawn to it from the porous metal due tothe higher flow of dissolved tin debris (and Galinstan) from thislocation. Conversely, a location which receives little tin debris willexperience little flow of dissolved tin debris (and Galinstan) and willtherefore have less Galinstan drawn to it. The use of porous metal inthis manner is not limited to Galinstan; porous metal may for example beused to deliver liquid fuel or some other metal or alloy.

Other alloys or metals which are liquid at room temperature may be usedinstead of Galinstan. For example Mercury could be used. Galinstanprovides advantages over Mercury in that it is non-toxic and is capableof dissolving more tin than Mercury.

Although described in the context of vanes of the housing, Galinstan, orsome other alloy or metal which is liquid at room temperature, may beused to capture tin (or other fuel) at any suitable location.

FIG. 14 shows schematically in cross-section the obscuration bar 60shown in FIGS. 3 and 4. The optical axis O of the radiation source isalso shown in FIG. 14. As may be seen from FIG. 14, the obscuration bar60 intersects with the optical axis O. The obscuration bar 60 comprisestwo faces 62 upon which radiation emitted from the laser LA (see FIGS. 3and 4) is incident. The faces 62 may be arranged to reflect the laserradiation away from the intermediate focus IF of the radiation sourceSO. The faces 62 may in addition arranged such that they do not directlyreflect laser radiation back into the laser LA (such direct reflectioncould give rise to instability in the operation of the laser). The faces62 of the obscuration bar 60 join together to form an edge 64. The edge64 may intersect with the optical axis O of the radiation source.

As mentioned above, the obscuration bar 60 blocks laser radiationemitted by the laser LA. In addition, the obscuration bar 60 blocks EUVradiation emitted from the plasma 14. Furthermore, residual tin dropletswhich remain after the plasma 14 has been formed will be incident uponthe obscuration bar 60. The obscuration bar 60 includes grooves 66 whichreceive the residual tin and direct it towards a gutter, drain or otherreceptacle.

The obscuration bar 60 may be formed for example from molybdenum inorder to be able to withstand high temperatures (or may be formed fromsome other suitable material). The bar may be heated to a temperaturewhich is above the melting temperature of tin (e.g., to above around200° C., e.g. to above around 230° C.), such that residual tin dropletswhich are incident upon the obscuration bar remain in liquid form. Aresidual tin droplet which is incident upon the obscuration bar 60 mayfor example initially remain stationary on the obscuration bar. Overtime, additional residual tin droplets may be incident at the same oradjacent locations on the obscuration bar, and these tin droplets maycoalesce to form a larger droplet. Once the tin droplet grows to aparticular size, gravity will cause the tin droplet to move downwards onthe obscuration bar. The tin droplet will move downwards until itreaches a groove 66, whereupon further downward movement of the tindroplet will be prevented by that groove. If the grooves 66 were notpresent, then there would be a risk that the droplet would continue toflow downwards on the obscuration bar 60 and would fall from the edge 64of the obscuration bar. This may be undesirable because it may lead tocontamination of the collector optic CO, or other surfaces of theradiation source SO.

Referring to FIG. 3, in an embodiment the obscuration bar 60 may have anon-horizontal orientation. That is, in FIG. 3 the obscuration bar 60may be oriented such that it is not parallel with the plane of the paperof FIG. 3 but instead is rotated about the optical axis O such that oneend of the obscuration bar is raised above the plane of the paper andone end of the obscuration bar is below the plane of the paper. Theobscuration bar 60 may thus be inclined relative to the horizontal. As aresult of the non-horizontal orientation of the obscuration bar 60,liquid tin in the grooves 66 will flow along the grooves due to gravity,and will flow to a lowermost end of the obscuration bar. The liquid tinmay then be received in a drain, gutter or some other suitablereceptacle.

The grooves 66 may have any suitable cross-sectional shape. The groovesmay for example be substantially v-shaped in cross-section (asillustrated in FIG. 14). The grooves may have a cross-sectional sizeand/or shape which gives rise to capillary action. The grooves may havea cross-sectional size and/or shape which gives rise to wicking actionwhich draws liquid fuel into the grooves.

The obscuration bar 60 may include a heating system which is arranged toheat the obscuration bar to a temperature above the melting temperatureof tin (e.g., to above around 200° C., e.g. to above around 230° C.).This may be considered to be an active heating system. Alternatively,the obscuration bar 60 may be heated by receiving heat from the plasma14 and the laser beam emitted by the laser LA. This may be considered tobe passive heating. The obscuration bar 60 may be heated by acombination of active and passive heating.

As will be appreciated from FIG. 14, a residual tin droplet which isincident upon a lowermost portion 68 of the obscuration bar 60 will notflow into a groove 66 because it is located beneath the lowermostgrooves on the obscuration bar. In order to prevent liquid tin fromdripping from the lowermost region 68 of the obscuration bar, thisportion may be heated to a temperature which is above the evaporationtemperature of tin. This temperature depends upon the pressure in theradiation source SO, and may for example be in the range 1100° C.-1600°C.

In an embodiment, heating of the lowermost portion 68 of the obscurationbar 60 to a temperature which causes evaporation may for example beachieved, at least in part, by arranging the width W of the connectionbetween the lowermost portion 68 and the remainder of the obscurationbar 60 to be sufficiently narrow that transfer of heat away from thelowermost portion 68 to the remainder of the obscuration bar is limited.The lowermost portion 68 will be heated by laser radiation emitted bythe laser LA (see FIGS. 3 and 4). Limiting the flow of heat from thelowermost portion 68 will limit the extent to which heat delivered tothe lowermost portion by the laser radiation can be conducted away fromthe lowermost portion, thereby raising the temperature of the lowermostportion to a temperature which is significantly higher than thetemperature of the remainder of the obscuration bar 60. The width W ofthe connection between the lowermost portion 68 and the remainder of theobscuration bar 60 may be selected such that the lowermost portion isheated to a temperature which is above the evaporation temperature oftin during operation of the radiation source.

The narrow connection between the lowermost portion 68 and the remainderof the obscuration bar is an example of thermal isolation of thelowermost portion. Other forms of thermal isolation may be used. Forexample, a material which is a poor thermal conductor may be used toconnect the lowermost portion to the remainder of the obscuration bar,thereby limiting the transfer of heat. The term “thermal isolation” isnot intended to mean that there is no transfer of heat between thelowermost portion and the remainder of the obscuration bar. Instead,“thermal isolation” may be interpreted as meaning that the transfer ofheat is limited such that there may be a significant temperaturedifference between the lowermost portion and the remainder of theobscuration bar.

The grooves 66 may have a width which is comparable to the expecteddiameter of tin droplets which will be received in the grooves. Thegrooves 66 may for example have a width of a few millimeters (e.g., lessthan 10 mm).

The obscuration bar, which may be referred to simply as a bar, may haveany suitable shape which allows grooves to be provided that direct theflow of liquid tin under the influence of gravity. The bar may forexample have more than two faces.

An alternative approach to using grooves would be to maintain the entireobscuration bar 60 at a temperature which is above the evaporationtemperature of tin. However, this may be undesirable because theevaporated tin could travel to and condense upon surfaces in theradiation source SO (or elsewhere), such as optical sources. Although inembodiments of the invention evaporation of tin takes place at thelowermost portion 68 of the obscuration bar, this is a relatively smallproportion of the obscuration bar, and the amount of evaporated tin isthus relatively limited.

The obscuration bar 60 may incorporate one or more features of otherembodiments of the invention.

One or more of the above embodiments may avoid the need to periodicallyinterrupt operation of the radiation source SO to remove tincontamination. Alternatively, one or more of the above embodiments mayincrease the period between interruptions of operation of the radiationsource SO to remove tin contamination.

Although in relation to one or more of the above embodiments referencehas been made to liquid tin, the embodiments may also be applicable toother liquids. For example, in relation to the embodiments described inconnection with FIGS. 11 to 13, if a liquid other than liquid tin isused to generate the EUV emitting plasma, that liquid may be used tomaintain the wetting nature of the debris receiving surfaces.

The liquid fuel which is used to generate the EUV emitting plasma may bethe same as the liquid fuel which is delivered onto the fuel debrisreceiving surface. Alternatively, fuel which is used to generate the EUVemitting plasma may be different from liquid fuel which is deliveredonto the fuel debris receiving surface. Where a different liquid fuel isdelivered to the fuel debris receiving surface, that fuel may be a fuelwhich provides a good wetting surface that promotes flow of liquid fueldebris. The liquid fuel may also be a fuel which reduces or avoidssplashing of liquid fuel debris from the fuel debris receiving surface.

Different features of different embodiments of the invention may becombined together. For example, the vanes 42 shown in FIG. 12 may beused to direct the flow of tin without a liquid tin inlet 48 beingpresent.

The laser LA may be separate from the radiation source SO, for examplebeing provided at a different location (the laser beam may be guidedinto the radiation source using a beam guide). Where this is the case,the laser LA may be considered not to form part of the radiation sourceSO.

Although the collector optic CO shown in FIGS. 3 and 4 is a singlecurved mirror, the collector may take other forms. For example, thecollector may be a Schwarzschild collector having two radiationcollecting surfaces. In an embodiment, the collector may be a grazingincidence collector which comprises a plurality of substantiallycylindrical reflectors nested within one another. The grazing incidencecollector may be suited for use in a discharge produced plasma (DPP)source. Embodiments of the invention may comprise a DPP source.

In the above description the fuel which is emitted from the fuel dropletemitter 10 is tin. However, other fuels, such as for example xenon orlithium may be emitted from the fuel droplet emitter 10 (and/or may beused to provide a wetting surface on a fuel debris receiving surface).

The term “vane” may be interpreted as meaning a protuberance. The term“vane” may be interpreted as meaning a blade, a plate or other thin,flat or curved object attached (e.g., radially) to a surface thatredirects the flow of a fluid, thereby providing directional control ofdrainage flow, and thus providing a fuel debris mitigation function(e.g., receives droplets of fuel and thereby reduces the likelihood offuel droplets being incident upon optical surfaces).

A vane may be fixed to the surface or removable (i.e., extractable froma surface where it is fixed in order to allow it to be cleaned orreplaced with a new vane).

The grooves may for example be formed by cutting into a surface (e.g., asmooth surface). Alternatively, the grooves may for example be formed byadding ribs or other structures to a surface such that grooves areestablished between the ribs or other structures.

FIG. 15 shows schematically viewed from above a portion of a liquid fueldebris guiding apparatus 102. The liquid fuel debris guiding apparatus102 may, for example, be immobile. The liquid fuel debris guidingapparatus may, for example, be a wall or part of a wall of the radiationsource SO, or may be some other part of the radiation source (or may beat some other location in the lithographic apparatus).

The liquid fuel debris guiding apparatus 102 comprises a plurality ofelectrodes 100. As depicted in FIG. 15, the electrodes 100 arephysically separated from each other, the electrodes being arranged withgaps between them which define paths on a surface of the liquid fueldebris guiding apparatus 102. Potential differences between electrodes100 may be set to guide liquid fuel debris 110 over the surface ofliquid fuel debris guiding apparatus 102, along the paths defined by thegaps between electrodes 100.

FIG. 16 is a cross sectional view along line A-A of a portion of theliquid fuel debris guiding apparatus 102. The liquid fuel debris guidingapparatus 102 includes two layers of supporting material 101, which mayfor example be glass (or any other suitable material). The electrodes100 are provided between the two layers of supporting material 101. Theelectrodes 100 may be formed from any conducting material. Theelectrodes may for example be layers of indium tin oxide and may forexample be delivered onto a first layer of supporting material 101 bychemical vapour deposition. Gaps between the electrodes 100 may, forexample, be created using an etching process. A layer of supportingmaterial 101 may then be provided on top of the electrodes 100.

A layer of insulating material 103 is provided on top of the supportingmaterial 101. The layer of insulating material 103 prevents currentflowing between electrodes 100 and liquid fuel debris on the surface ofthe liquid fuel debris guiding apparatus 102 (which could occur if thesupporting material 101 was not sufficiently insulative). The insulatingmaterial may, for example, be polytetrafluoroethylene (PTFE-brand nameTEFLON® by DuPont Co.). The supporting material 101 may itself be aninsulating layer, in which case a separate insulating layer may not berequired.

The electrodes 100 are each electrically connected to one terminal ofvoltage sources 105. The voltage sources 105 are connected at theirother terminals to common reference voltages 106, which may for examplebe earth (as depicted in FIG. 16). The voltage sources 105 may providean AC voltage. The voltage sources 105 may alternatively provide a DCvoltage.

Liquid fuel debris guiding apparatus 102 may be tilted such that anupper end 107 of the liquid fuel debris guiding apparatus is elevatedabove a lower end 108 of the liquid fuel debris guiding apparatus. As aresult, a droplet of liquid fuel debris 110 positioned on the surface ofthe liquid fuel debris guiding apparatus will move over the surface ofthe liquid fuel debris guiding apparatus 102, under the influence ofgravity, in the direction depicted by arrow 109 in FIG. 15.

The voltage sources 105 may be set such that a potential differenceexists between two electrodes. When a potential difference existsbetween electrodes, a potential well is formed in the proximity of thegap between electrodes. If a droplet of liquid fuel debris were to belocated away from the gap between the electrodes, then the droplet ofliquid fuel debris would experience no electric field. However, if thedroplet of liquid fuel debris 110 were to overlap the gap betweenelectrodes then it would experience an electric field (arising from thepotential difference between the electrodes). When a droplet of liquidfuel debris experiences an electric field, electric field lines passthrough the droplet of liquid fuel debris and the potential energy ofthe droplet of liquid fuel debris is reduced (compared with an identicaldroplet of liquid fuel debris which experiences no electric field).Referring to FIG. 15, paths on the surface of the liquid fuel debrisguiding apparatus defined by gaps between electrodes 100 a-e thereforerepresent positions at which the potential energy of a droplet of liquidfuel debris can be reduced by setting the voltages of electrodes 100 a-esuch that potential differences exist between them.

FIG. 17 shows schematically viewed from above the same portion of aliquid fuel debris guiding apparatus 102 that is depicted in FIG. 15.However, in FIG. 17, electrode 100 b is supplied with a voltage of 400Vand all other electrodes are connected to earth. A droplet of liquidfuel debris 110 positioned on the surface of liquid fuel debris guidingapparatus 102 in the vicinity of the gap between electrodes 100 a and100 b will be held in the gap by the potential well established by thepotential difference between the electrodes. The droplet of liquid fueldebris moves over the surface of the liquid fuel debris guidingapparatus, under the influence of gravity, in the direction depicted byarrow 109 a. Because the potential difference between electrodes 100 aand 100 b reduces the potential energy of the droplet of liquid fueldebris 110, the droplet does not deviate to the left or right but isguided along the path defined by the gap between the electrodes.

The droplet of liquid fuel debris 110 travels in the direction indicatedby arrow 109 a until it arrives at the top of electrode 100 d. Thedroplet is thus at a junction and can either move to the left or to theright. The droplet will move in the direction which minimises itspotential energy. If the potential difference between electrodes 100 band 100 d is sufficiently large that the potential energy of a dropletof liquid fuel debris positioned at the top of electrode 100 d isminimised along the path defined by the gap between electrodes 100 b and100 d, then the droplet of liquid fuel debris will follow the pathdefined by the gap between electrodes 100 b and 100 d (depicted byarrows 109 b and 109 c).

Similarly, if the potential difference between electrodes 100 b and 100e is sufficiently large that the potential energy of a droplet of liquidfuel debris positioned at the top of electrode 100 e is minimised alongthe path defined by the gap between electrodes 100 b and 100 e, then thedroplet of liquid fuel debris will follow the path depicted by arrows109 d and 109 e.

Alternatively, electrode 100 e may be supplied with a voltage (forexample 400V) such that the potential difference between electrodes 100e and 100 d is sufficiently large that the potential energy of a dropletof liquid fuel debris positioned at the top of electrode 100 e isminimised along the path defined by the gap between electrodes 100 e and100 d. Accordingly, the droplet of liquid fuel debris will follow thepath defined by the gap between electrodes 100 e and 100 d, as depictedby arrows 109 f and 109 g.

More generally, the voltages of any electrodes 100 a-e may be set so asto create potential differences between electrodes such that liquid fueldebris follows any desired path, defined by gaps between electrodes,over the surface of the liquid fuel debris guiding apparatus.

The path of liquid fuel debris over the surface of a liquid fuel debrisguiding apparatus 102 may be determined by setting potential differencesbetween electrodes such that the potential energy of liquid fuel debrisis minimised by following a desired path. The potential differencebetween electrodes, where the gap between electrodes defines a desiredpath of liquid fuel debris, may be set to any potential difference whichis sufficient to minimise the potential energy of the liquid fuel debrisalong the path defined by the gap between electrodes. A sufficientpotential difference may for example be 400V. Any other suitablepotential difference may be used. The path defined by the gap betweenelectrodes may be seen as an electrical equivalent of a groove asdescribed above, wherein the flow is controlled, in addition to gravity,also by the electrical field created by the potential difference appliedbetween the electrodes.

Voltage sources used to apply voltages to the electrodes may becontrolled by a controller (not pictured). The controller may be anyapparatus configured to selectively apply one or more voltages toelectrodes, thereby establishing a potential difference between at leasta first electrode and a second electrode. The controller may for exampleinclude a processor.

In general, any suitable voltage source may be used to apply a voltageto one or more electrodes.

In an embodiment, the voltages may be selectively applied to electrodes.That is, voltages may be applied to and disconnected from theelectrodes, for example in order to select different paths along whichthe liquid fuel debris is guided. Alternatively, the voltages may befixed, e.g. such that the same potential difference always existsbetween two electrodes when the fuel debris guiding apparatus isoperational.

The liquid fuel debris may be liquid tin, or may be some otherconducting liquid.

FIG. 18 shows schematically in cross section a fuel collector (80)according to an embodiment of the invention. The fuel collector 80 mayfor example correspond with the fuel tin collector 46 shown in FIG. 11.The fuel collector may be provided at any suitable location in theradiation source SO. The fuel collector may be positioned to receiveliquid fuel (e.g. liquid fuel debris) from a gutter or from othersources.

The fuel collector comprises a receptacle 81 and a reservoir 82. Liquidfuel 79 is received in the reservoir 82 of the fuel collector. Thereservoir 82 is provided with a hole 83 through which liquid fuel maydrain from the reservoir into the receptacle. The liquid fuel 79 may betin or may be some other fuel.

The receptacle is formed from a base 84 and walls 85. The receptacle 81may for example be cylindrical, rectangular, or may have any suitableshape. The receptacle may for example have a diameter of around 100 mm.The reservoir 82 is located above the receptacle 81. The reservoircomprises a base 86 and walls 87. The reservoir may be cylindrical,rectangular, or may have any suitable shape. The reservoir 82 may beseated on top of the receptacle 81 (e.g. as shown). The reservoir 82 maybe removable from the receptacle 81 in order to allow easy access to thereceptacle. Alternatively, the reservoir 82 may be raised above thereceptacle 81. The reservoir 82 may be heated in order to ensure thatthe fuel remains in liquid form whilst it is held in the reservoir. Thereceptacle 81 may be unheated, such that fuel will solidify in thereceptacle.

A raised lip 88 extends around the hole 83. The raised lip 88 acts toprevent liquid fuel 79 from passing into the hole 83 until a level ofthe liquid fuel exceeds the height of the raised lip. The raised lip isformed from a non-wetting material (e.g. a material which provides acontact angle between the liquid fuel and the surface that is greaterthan 90°). The raised lip 88 may have a rounded upper surface (e.g. witha radius of curvature of 1 mm or more). The raised lip 88 may forexample be formed from molybdenum.

Cohesive forces within the liquid fuel 79 will act to prevent the liquidfuel from flowing over the raised lip and into the hole 83 when thelevel of the liquid fuel only marginally exceeds the height of theraised lip (e.g. if the height is exceeded by around 1 mm).

The level of the liquid fuel 79 will continue to rise above the top ofthe raised lip 88 until such time that the cohesive forces within theliquid are not sufficient to prevent liquid fuel from flowing over theraised lip and into the hole 83 (e.g. when the level of the liquid fuelexceeds the liquid fuel's capillary height). When this occurs asubstantial amount of liquid fuel will flow into the hole 83 and intothe receptacle 81. The flow of liquid fuel into the hole 83 may be acontinuous flow, and may be referred to as a single flow. The liquidfuel may flow into the hole 83 until the level of the liquid fuel isbelow the height of the raised lip 88.

The volume of liquid fuel which passes into the receptacle 81 during asingle flow is significantly greater than the volume of liquid fuel thatwould enter the receptacle if the receptacle were to directly receive adroplet of liquid fuel from a gutter. Because the receptacle 81 is notheated, the liquid fuel will solidify in the receptacle 81. If thereceptacle were to receive droplets of liquid fuel (e.g. from a gutter)then this would lead to the formation of a stalagmite extending upwardlyfrom the base 84. Thus, the potential fuel storage capacity of thereceptacle 81 would not be utilised, and the receptacle would need to bereplaced frequently. When the embodiment of the invention is used, theamount of liquid fuel entering the receptacle 81 as a single flow isvery much larger than a droplet, and the liquid fuel spreads out on thebase 84 of the receptacle 81 before it solidifies. The fuel solidifiesas a relatively flat body 78, which may resemble a pancake. In this waythe volume of the receptacle 81 is more efficiently used, meaning thatthe intervals between replacing the receptacle are increased. This mayadvantageously increase the period of time for which the radiationsource SO may be operated.

The height of the raised lip 88 may for example be equal to or greaterthan the capillary length of the liquid fuel (e.g. liquid tin). This isadvantageous because it increases the amount of liquid fuel which willenter the receptacle 81 in a single flow (compared with the situation ifthe raised lip were to have a height significantly less than thecapillary length of liquid fuel). The capillary length of liquid tin is3 mm, and thus the height of the raised lip 88 may be 3 mm or more. Inthis context, the height of the raised lip 88 is as measured from thesurface of the base 86 of the reservoir 82.

The walls 87 of the reservoir may for example have a height whichexceeds the height of the raised lip 88 by at least the capillary lengthof the liquid fuel. This is because it is expected that the level of theliquid fuel 79 will not exceed the height of the raised lip 88 by morethan a capillary length (flow of the liquid fuel into the hole 83 willoccur when the liquid fuel reaches this height).

Although it may be possible to form the raised lip 88 without a roundedupper surface, doing so would tend to promote the flow of liquid fuelinto the hole 83 when the level of the liquid fuel has reached a lowerlevel (compared with the case when the raised lip has a rounded uppersurface). This could occur for example due to a sharp edge of the lipcausing ribbon of liquid fuel to flow across the raised lip.Consequently, the volume of liquid fuel entering the receptacle in asingle flow would be reduced if the raised lip did not have a roundedupper surface.

Similarly, although the raised lip 88 may be made from a wettingmaterial instead of a non-wetting material, this would cause the liquidfuel to flow more easily into the hole 83, thereby reducing the volumeof liquid fuel entering the receptacle in a single flow.

Although molybdenum is mentioned as the material from which the raisedlip 88 is formed, the raised lip may be formed from any suitablematerial. The material may for example be a non-wetting material.

In the illustrated embodiment, in addition to the raised lip 88 the hole83 is also provided with a downwardly projecting lip 89. The downwardlyprojecting lip acts to prevent liquid fuel from flowing from the exit ofthe hole 83 onto a lower surface of the base 86. This would not bedesirable because the liquid fuel could remain on the base 86 for sometime and then subsequently drip into the receptacle 81, thereby leadingto the formation of stalagmite.

The downwardly projecting lip 89 has a sharp edge, which inhibits theflow of liquid fuel around the downwardly projecting lip and onto thebottom of the base 86. The downwardly projecting lip 89 may for exampleextend by 2 mm or more beyond the bottom surface of the base 86.

The downwardly projecting lip 89 may be formed from any suitablematerial.

The downwardly projecting lip 89 may be formed from a non-wettingmaterial. The downwardly projecting lip 89 may be formed frommolybdenum.

The downwardly projecting lip 89 and the raised lip 88 may be formedfrom a single piece of material (e.g. formed as tube), which may befitted to the base 86. An outer surface of the tube may include a step,thereby allowing the tube to be pushed into the hole 83 and securelyretained therein.

The hole 83 may for example have a diameter of 8 mm or more. Forming thehole 83 with a diameter of 8 mm or more may reduce the likelihood of thehole becoming clogged (compared with a hole with a narrower diameter).The hole may for example have a diameter of up to 12 mm.

The hole 83 may for example be circular or elliptical in shape. The hole83 may have any suitable shape.

FIG. 19 shows schematically viewed from one side in partialcross-section a radiation source housing apparatus 141 according to anembodiment of the invention. The radiation source housing apparatus 141comprises a rotatably mounted housing 140. The illustrated housing 140is substantially conical, although it may have any suitable shape. Aplurality of vanes 142 extend inwardly from the housing 140. The vanes142 may be distributed around the housing 140. Although seven vanes areshown in FIG. 19, this is merely a schematic illustration, and anysuitable number of vanes may be provided. For example, twenty or morevanes, forty or more vanes or sixty or more vanes may extend inwardlyfrom the housing 140. A fuel debris collector 146 is located beneath alowermost portion of the housing 140.

The radiation source housing apparatus further comprises a heater 150provided on one side of the housing 140. The heater 150 is arranged toheat a portion of the housing which is in the vicinity of the heater toa temperature which is above the melting temperature of tin. Forexample, the heater 150 may be arranged to hold the temperature of theportion of the housing 140 in the vicinity of the heater to atemperature above around 230° C., e.g. above around 250° C.

The radiation source housing apparatus further comprises a cooler 152.The cooler 152 is shown in FIG. 19 as being on an opposite side of thehousing 140 from the heater 150. The cooler 152 may be arranged to coola portion of the housing 140 in the vicinity of the cooler such thatthat portion of the housing has a temperature which is below the meltingpoint of tin. For example, the cooler 152 may keep the temperature ofthe portion of the housing 140 in the vicinity of the cooler belowaround 230° C. (e.g. below around 200° C.).

The radiation source housing apparatus further comprises an actuator 154connected to the housing 140. The actuator causes the housing 140 torotate. The housing 140 may for example be mounted such that it rotatesabout the optical axis OA of the radiation source. The rotation may forexample be at a speed of around one rotation per hour (or may be at anyother suitable speed). The rotation may be continuous or may beintermittent. For example, in an embodiment the housing 140 may rotate30° after 10 minutes has elapsed, rotate another 30° after another 10minutes has elapsed, etc. In this embodiment, a full rotation of thehousing 140 will take place in two hours. Any suitable amounts ofrotation and intervals may be used.

The actuator 154 may for example be a motor, or may for example comprisea connection to a remotely located motor. The rotatable mounting of thehousing 140 may take any suitable form, and for simplicity is not shownhere. If the rotatable mounting is located in a part of a radiationsource which is at vacuum, then a vacuum compatible rotatable mountingmay be used.

The radiation source housing apparatus 141 forms part of a radiationsource. Other parts of the radiation source are not shown, but may forexample correspond with parts of the radiation source SO shown in FIGS.3 and 4. The radiation source may use tin, or any other suitable fuel.In this description tin is referred to, but embodiments of the inventionmay be adapted for any suitable fuel.

FIG. 20 shows schematically the housing 140 in cross-section along lineB-B. The vanes 142 may be seen extending inwardly from the housing 140,the vanes having been represented schematically with triangularcross-sectional shapes. Although twelve vanes are shown, any suitablenumber of vanes may be provided. The vanes may for example be arrangedsuch that there is little or no spacing between them. The heater 150 isschematically represented as being rectangular in cross-section, but mayhave any suitable shape. As may be seen from FIG. 20, the cooler 152extends around more than three quarters of the circumference of thehousing. The cooler 152 is configured such that the cooler cools themajority of the housing 140. The heater 150 extends around less than onequarter of the circumference of the housing 140. The heater 150 isconfigured such that the heater heats less than one quarter of thecircumference of the housing 140.

Rotation of the housing caused by the actuator 154 (see FIG. 19) isrepresented schematically by an arrow in FIG. 20. The housing may rotatein the opposite direction to that shown.

Operation of the apparatus shown in FIGS. 19 and 20 is as follows. Thecooler 152 cools the portion of the housing 140 around which it extendsto a temperature which is below the melting temperature of tin. When tindebris is incident upon the cooled portion of the housing 140 ittherefore solidifies. This is advantageous because it reduces thelikelihood of tin dripping from the housing 140 and on to opticalcomponents of the radiation source such as the collector optic CO (seeFIGS. 3 and 4).

Tin debris which is incident upon the cooled portion of the housing 140will remain in a solid state because it is below the melting temperatureof tin. This will remain the case until the rotation of the housing 140caused by the actuator 154 brings the debris bearing portion of thehousing into the vicinity of the heater 150. The heater 150 raises thetemperature of the portion of the housing adjacent the heater until thatportion of the housing 140 is above the melting temperature of tin. Thetin debris thus melts and becomes liquid as it approaches the heater150. Once the tin debris has become liquid, it flows down the housing140 under the influence of gravity, and is received in the tin collector146.

The housing 140 continues to turn due to the actuator 154. The heatedportion of the housing 140 will, as it moves away from the heater 150,cool to a temperature which is below the melting temperature of tin(thus becoming a cooled portion of the housing). Once this has happened,any tin debris which is incident upon that portion of the housing willsolidify and will therefore not drip onto the collector or other opticalsurfaces.

As will be understood from the above, a first portion of the housing 140is heated by the heater 150 and a second different portion of thehousing is cooled by the cooler 152. Due to rotation of the housing 140the heated portion will move from being adjacent to the heater 150 tobeing adjacent to the cooler 152, and the heated portion will thereforebecome a cooled portion. Similarly, due to the rotation of the housing140 part of the cooled portion will move from being adjacent to thecooler 152 to being adjacent to the heater 150, and that part of thecooled portion will therefore become a heated portion.

As may be seen from FIGS. 19 and 20, the heater 150 is positionedadjacent to a portion of the housing 140 which does not have adownwardly facing inner surface. The heater 150 is positioned adjacentto a portion of the housing 140 where vanes 142 do not includedownwardly facing surfaces. The heater 150 may be described as beingpositioned adjacent to a lowermost side of the housing 140.

As a result of the positioning of the heater 150, there is nopossibility of tin which is melted by the heater dripping off adownwardly facing surface in an unwanted manner (e.g. onto the collectoroptic CO or some other optical surface). Instead, the liquid tin isconstrained to flow downwards under the influence of gravity along anupwardly facing inner surface of the housing 140 (e.g. includingupwardly facing vanes 142). The liquid tin flows downwards along theupwardly facing portion of the housing 140 until it reaches a bottom endof the housing, from where it drips into the tin collector 146. In thiscontext the term “upwardly facing” is not intended to be limited to asurface which faces vertically upwards, but is instead intended toencompass a surface which faces in a direction that includes avertically upward component. Similarly, in this context the term“downwardly facing” is not intended to be limited to a surface whichfaces vertically downwards, but is instead intended to encompass asurface which faces in a direction that includes a vertically downwardcomponent.

In an embodiment, the portion of the housing 140 adjacent to the heater150 does not have an inner surface from within which liquid tin candrip. In other words, there is no location within that portion of theinner surface at which liquid tin may accumulate and form into a dropletwhich falls from the inner surface. Liquid tin may accumulate and forminto a droplet at a lowermost edge of the portion of the inner surface(falling from there into the tin collector 146). However, dripping ofliquid tin in this manner is from the lowermost edge of the innersurface and is therefore not from within the inner surface.

In an embodiment, the heater is arranged to heat the first portion ofthe rotatably mounted housing to a temperature at which tin (or otherfuel) on the housing will be in a liquid state. In an embodiment, thecooler is arranged to cool the second portion of the rotatably mountedhousing to a temperature at which tin (or other fuel) on the housingwill be in a solid state.

The term “solid state” may be considered to encompass a state such as agel-like state where the tin (or other fuel) cannot flow or drip but hassome malleability.

The vanes 142 may provide some guidance of the flow of liquid tin. Thisguidance of the flow of the liquid tin by the vanes may not benecessary, since the position of the heater 150 is such that flow causedby gravity will be in a desired direction (e.g. towards the tincollector 146). The vanes 142 may however provide other usefulfunctionality, such as reducing the likelihood of incident tin debrisscattering from the housing 140 towards a collector optic CO or otheroptical surface.

Grooves may be provided in the vanes 142 and/or in portions of thehousing 140 between vanes. The grooves may not be needed to guide theliquid tin in particular desired directions. However, the grooves maynevertheless provide a degree of guidance.

An advantage of the embodiment of the invention is that it reduces oreliminates dripping of liquid tin onto the collector optic CO or otheroptical surfaces of the radiation source SO. Another advantage is thatthe apparatus is cooler than would be the case if the entire housingwere to be heated (which may for example be necessary for otherembodiments). This may be useful for example if it is desired to performmaintenance or repair work upon the radiation source, because theradiation source will more rapidly cool to a temperature which issufficiently low to allow the apparatus to be safely accessed.

The heater 150 may for example extend around approximately one-tenth orless of the circumference of the housing 140. The heater 150 may forexample extend around up to one-sixth of the circumference of thehousing 140. The heater 150 may for example extend around less thanone-third of the circumference of the housing 140. The extent to whichthe heater 150 extends around the circumference of the housing 140 willat least partially depend upon thermal properties of the housing (e.g.the extent to which heat that is delivered to the housing is conductedaround the housing). The heater may extend sufficiently far around thecircumference that tin debris is heated for a sufficiently long timethat it becomes liquid and is able to flow into the tin collector 146(e.g. rather than solidifying again before it reaches the tincollector). This may depend at least partially upon the size of tindebris particles which are expected to be received on the housing (alarger tin debris particle will require more heat before it melts).

In an embodiment, the heater does not extend so far around the housingcircumference that it keeps the tin in liquid form after the housing hasrotated to a point at which liquid tin could drip from the housing ontoan unwanted location (e.g. onto the collector optic CO). For example,the heater 150 may provide sufficient heat to keep tin in liquid formaround less than one-third of the circumference of the housing 140, lessthan one-sixth of the circumference of the housing, or around one-tenthor less of the circumference of the housing.

The cooler 152 may for example extend around at least two thirds of thecircumference of the housing 140. The cooler 152 may for example extendaround up to five sixths of the circumference of the housing 140. Thecooler 152 may for example extend around up to nine tenths of thecircumference of the housing 140.

The housing may for example be formed from steel or molybdenum, or anyother suitable material. The term “formed from” is not intended to meanthat all other materials are excluded; the housing may include othermaterials. Where steel is used, the steel may for example be coated withtin. This may for example promote sticking of the tin debris to thehousing. Similarly, molybdenum may be provided with a tin coating.

The thermal properties of steel are such that although some heat will beconducted from the heater 150 around the housing 140, the conducted heatwill be relatively limited. Thus, an elevated housing temperature causedby the heater 150 may be primarily in the vicinity of the heater.Although the thermal conductivity of molybdenum is greater than that ofsteel, in an embodiment in which molybdenum is used, the elevatedhousing temperature may still be generally restricted to the vicinity ofthe heater 150.

The thickness of the housing 140 may be chosen taking into accountthermal properties of the housing. A thinner housing will provide lessheat conduction than a thicker housing (for a given temperature in thevicinity of the heater).

In the above described embodiment, the heater 150 and cooler 152 act ononly part of the housing 140 (the illustrated rotationally mountedhousing). The radiation source housing may however include otherportions, for example other walls as shown in FIGS. 3 and 4. In anembodiment, one or more of these other portions may also be heated andcooled using appropriately positioned heaters and coolers, and theseportions may also be driven to rotate (e.g. using an actuator). In anembodiment, the entire housing may be driven to rotate (and may beheated and cooled accordingly). Where this is the case, the rotatablemounting may be located outside of the part of the radiation sourcewhich is at vacuum, and a rotatable mounting which is not vacuumcompatible may be used.

In FIG. 20 the heater 150 is centred at the lowermost point of thecircumference of the housing 140. However, in an embodiment, the heater150 may be partially offset, for example extending further in anupstream direction from the lowermost point and extending less far in adownstream direction from the lowermost point (the term “upstream” isintended to mean in a direction opposite to the direction of movement ofthe housing). This is illustrated schematically by dashed line 150 a.Where this is arrangement is used, heating of solid tin debris may beginearlier and may cease earlier. This may increase the likelihood of tindebris having melted to liquid form when that tin debris is at thelowermost point of the circumference of the housing 140. It may reducethe likelihood of tin debris being in liquid form when it has moved asignificant distance away from the lowermost point of the circumferenceof the housing 140.

FIG. 21 shows schematically in cross section a fuel collector 90according to an embodiment of the invention. The fuel collector 90 mayfor example correspond with the tin collector 46 shown in FIG. 11 or thetin collector 146 shown in FIG. 19. The fuel collector may be providedat any suitable location in the radiation source SO. In use, the fuelcollector 90 is positioned to receive liquid fuel (e.g. liquid fueldebris such as tin) from a fuel source 91 (e.g. a drain or gutter whichforms part of an EUV radiation source).

The fuel collector comprises a receptacle 92. The receptacle 92 isformed from a base 92 a and one or more walls 92 b and is provided withan open end 93 and a closed end 94. In the illustrated embodiment, thereceptacle 92 is generally cylindrical in shape, however, the receptaclemay have any suitable shape.

In use, the receptacle is orientated with the open end 93 generallyabove the closed end 94. As shown, the receptacle 92 may be orientatedat an angle. However, the receptacle 92 may alternatively be disposed sothat the base 92 a is substantially horizontal. The open end 93 providesan entrance through which liquid fuel may enter the receptacle 92. Theclosed end 94 provides a storage portion of the receptacle.

In use, the receptacle is disposed generally below a liquid fuel source91. The liquid fuel may be tin or may be some other fuel. Fuel may fallunder gravity from the fuel source 91 in the direction indicated byarrow 95 into the receptacle 92 via the open end 93.

The receptacle further comprises a shelf 96. The shelf 96 is disposedwithin the receptacle 92 such that fuel falling from the fuel source 91is incident upon an upper surface 96 a of the shelf. Substantially allfuel falling from the fuel source 91 may be incident upon the uppersurface 96 a of the shelf.

The shelf 96 comprises a cantilever structure with one end of the shelf96 being supported by a wall 92 b of the receptacle 92, the shelf 96extending inwards from said wall 92 b.

The shelf 96 is not heated. Therefore the temperature of the shelf 96will generally be below the melting point of the fuel, which may, forexample, be tin. As a result, as fuel drips onto the shelf 96, it willsolidify and adhere to the upper surface 96 a, forming a pancake offuel. In this context the term “pancake” may be considered to meanhaving a form which visually resembles a pancake.

The shelf 96 may be formed from a non-wetting material. In particular,the shelf may be formed from a material which is low wetting withrespect to a fuel it is desired to collect. Advantageously, thisprevents the fuel from adhering strongly to the shelf and makes iteasier for solidified fuel to be removed from the surface of the shelf.In one embodiment of the invention, the fuel collector 90 isparticularly suitable for the collection of tin and the shelf 96 isformed from molybdenum. Molybdenum is chosen for the shelf 96 since itis low wetting with respect to tin (i.e. providing low adhesion forcesbetween the shelf and the tin).

When no fuel is on the shelf 96 it is generally disposed in a firstposition (as indicated by the solid line in FIG. 21). The first positionis such that, in use, the upper surface 96 a of the shelf 96 isgenerally horizontal.

The shelf is arranged so that as fuel builds up on the shelf, the weightof the fuel will cause the shelf 96 to rotate so that the angle ofinclination of the upper surface 96 a will increase (relative tohorizontal). Because the shelf is inclined relative to the horizontalthere will be a component of the weight of the pancake of solid fuelwhich acts along the upper surface, tending to pull the pancake of fueldownwards and towards a distal end 96 b of the shelf. This is resistedby the adhesive force between the fuel and the upper surface 96 a of theshelf. At some point, the shelf will reach a second position (asindicated by the dotted line in FIG. 21) wherein the component of theweight of the pancake of fuel is sufficient to overcome the adhesiveforces and the pancake will fall off shelf and into the storage portion.

Movement of the shelf 96 between the first and second positionstherefore provides a mechanism for transferring fuel from the uppersurface 96 a to the storage portion.

As will be appreciated by the skilled person, some other mechanism fortransferring a pancake of fuel from the upper surface 96 a to thestorage portion may be provided. For example, the shelf 96 may besubstantially fixed relative to the receptacle 92 and may be providedwith a member that is operable to sweep across the shelf 96 and push thepancake of fuel off the shelf and into the storage portion.

The shelf 96 may be arranged to move between the first and secondpositions in any suitable way. For example, the shelf may be rotatablyconnected to the wall 92 b of the receptacle 92, facilitating rotationbetween the first and second positions. Additionally or alternatively,the shelf 96 may be formed from an elastic material which will deformunder the weight of incident fuel.

The shelf 96 is provided with a resilient bias, which biases it towardsthe first position. The resilient bias may comprise a spring. In oneembodiment, the shelf 96 is formed from an elastic material andcomprises a leaf spring.

The resilient bias resists movement from the first position to thesecond position. Once the shelf 96 has reached the second position andpancake of fuel slides off the shelf 96 and into the storage portion,the resilient bias causes the shelf 96 to move back to the firstposition. The resilient bias may be tuned so as to alter the size ofpancakes of fuel that are created. For example, the stiffness of theshelf 96 may be tuned to achieve the desired size of fuel pancakes so asto ensure an acceptable level of filling. This may be achieved bychoosing the thickness of the shelf: a relatively thin shelf 96 will beless stiff and therefore may create a relatively small and thin pancakewhereas a thicker shelf 96 will be stiffer and may create a larger,heavier pancake.

The shelf 96 may for example comprise a Molybdenum plate which is around0.1 mm thick.

A fuel collector 90 of the type shown in FIG. 21 allows quantities offuel to be deposited in the storage portion periodically.Advantageously, this can prevent the formation of a single stalagmite inthe receptacle 92, and thereby lead to an improvement in the fillingrate of receptacle 92 (i.e. increase the time which elapses before thefuel collector is full). In turn, this has a significant impact on theavailability of an EUV radiation source, since it may not be possible tooperate the EUV radiation source whilst the fuel collector 90 is beingemptied.

The production of fuel, such as tin, from the fuel source 91 may be low.As such, the fuel may fall into the receptacle as individual drops. Forexample, 0.3 ml drops may fall from the fuel source at a rate of around3 drops per hour (although other amounts of fuel may fall into thereceptacle). This is equivalent to around 150 ml of fuel per week. Sincethe temperature of the collector 90 is generally below the melting pointof the fuel, drops of fuel will solidify on top of each other within thereceptacle. Therefore, without the shelf 96, a stalactite may be formedwith a rather small diameter. As a consequence, fuel would reach the topof the receptacle 92 after the supply of a relatively small quantity offuel, requiring the receptacle 92 to be emptied. Thus, the potentialfuel storage capacity of the receptacle 92 would not be utilized, andthe receptacle 92 would need to be emptied or replaced frequently.

When the embodiment of the invention is used, the shelf 96 prevents asingle fuel stalagmite from forming. Rather, small quantities of solidfuel are formed on the shelf 96 (e.g. in the form of pancakes), whichare subsequently transferred to a lower storage portion of thereceptacle. In this way the volume of the receptacle 92 is moreefficiently used, meaning that the intervals between replacing oremptying the receptacle are increased. This may advantageously increasethe period of time for which the radiation source SO may be operated.

In the above described embodiment of the invention, the shelf 96 is anobject that is disposed within the receptacle 92 such that fuel passingthrough the entrance 93 is incident upon a surface 96 a of the shelf 96.Movement of the shelf 96 between the first and second positions providesa mechanism for transferring fuel from the upper surface 96 a to thestorage portion. However, other objects may alternatively be used. Forexample, the object may comprise a wheel with a curved outer surface.The wheel may be disposed in such a way that fuel passing through theentrance 93 is incident upon its curved surface in such a way that solidfuel adhered thereto will cause the wheel to rotate about its axis. Asthe wheel rotates, more fuel may be deposited onto the curved surface atdifferent circumferential positions. As the wheel rotates, deposits ofsolid fuel on a lower portion of the wheel may fall off into the storageportion.

FIG. 22 shows schematically in cross section a fuel collector 100according to an embodiment of the invention. The fuel collector 100 mayfor example correspond with the tin collector 46 shown in FIG. 11 or thetin collector 146 shown in FIG. 19. The fuel collector may be providedat any suitable location in the radiation source SO. In use, the fuelcollector 100 is positioned to receive liquid fuel (e.g. liquid fueldebris) from a fuel source 101.

The fuel collector comprises a receptacle 102. The receptacle 102 isformed from one or more walls 102 a and is provided with a first end 103and a second end 104. In use the first end 103 end forms an entrancethrough which liquid fuel may enter the receptacle 102 and is orientatedgenerally above the second end 104. In use, the second end 104 forms anexit through which fuel may be removed from the receptacle 104. In theillustrated embodiment, the receptacle 102 is generally cylindrical inshape, however, the receptacle may have any suitable shape. A portion ofthe wall of the receptacle 102 comprises a surface 102 b which isarranged to form a slide (the wall portion is generally upward facingand inclined relative to the horizontal). The second end 104 is disposedat a lower end of the slide.

In use, the receptacle 102 is disposed generally below a liquid fuelsource 101. The liquid fuel may be tin or may be some other fuel. Fuelmay fall under gravity from the fuel source 101 in the directionindicated by arrow 105 into the receptacle 102 via the open end 103.Such liquid fuel is incident upon the receptacle wall surface 102 bwhich is arranged to form a slide.

The receptacle wall surface 102 b is formed from a material which is lowwetting with respect to a fuel it is desired to collect. Advantageously,this prevents the fuel from adhering strongly to this surface 102 b. Inone embodiment of the invention, the fuel collector 100 is particularlysuitable for the collection of tin and surface 102 b is formed frommolybdenum. Molybdenum is chosen since it is low wetting with respect totin (providing low adhesion forces between the surface 102 b and thetin).

The fuel collector 100 may be unheated. The temperature of at least thesurface 102 b upon which liquid fuel is incident is below the meltingpoint of a fuel it is desired to collect. Therefore, as the liquid fuelis incident upon the surface 102 b, it will solidify and adhere weaklythereto.

Such a fuel collector 100 allows quantities of fuel to be deposited inthe storage portion periodically. When liquid fuel is incident upon thesurface 102 b, it will solidify and will only weakly adhere to thesurface. Since the surface 102 b is inclined and arranged to form aslide, once a sufficient quantity of solid fuel builds up it will slidedown the surface 102 b due to gravity. That is, the weight of thequantity of solid fuel overcomes adhesion forces between the surface ofthe surface 102 b and the solid fuel, causing the solid fuel to slidedown the surface. Thus, fuel does not adhere to the receptacle, therebyallowing the receptacle to be more easily emptied.

The fuel collector 100 further comprises two vacuum valves 106, 107. Afirst valve 106 is disposed towards the first end 103 of the receptacle102. A second valve 107 is disposed towards the second end 104 of thereceptacle 102 and is operable to seal the exit of the receptacle 102.

During normal operation of the EUV radiation source the first valve 106is open, allowing fuel to enter the receptacle, and the second valve 107is closed, so that the receptacle 102 may be maintained at substantiallythe same pressure as the EUV radiation source (not shown).

The first and second valves 106, 107 form an air lock. Accordingly, inorder for fuel to be removed from the receptacle 102, the first valve106 is closed to isolate the air lock portion of the receptacle 102 fromthe EUV radiation source. Once the first valve 106 is closed, the secondvalve 107 is opened so as to allow fuel to be removed from thereceptacle. The surface 102 b upon which liquid fuel is incident isinclined, is not heated and is formed from a material which is lowwetting with respect to a fuel it is desired to collect (such as, forexample, tin). Thus, slices of solid fuel are formed which do not adherestrongly to the receptacle 102. Once they are sufficiently large, theywill fall under gravity down the surface 102 b towards the exit. Whenthe exit has been opened using valves 106, 107 the slices of solid fuelwill fall out of the receptacle.

In order for normal operation to resume, the second valve 107 is closed.Once the second valve 107 is closed, the air lock is pumped down to forma vacuum. The first valve 106 is then opened and normal operation of thefuel collector 100 may resume.

Although the air lock formed by valves 106, 107 is described in relationto the fuel collector shown in FIG. 22, the air lock may be used withany other suitable fuel collector (e.g. the fuel collector shown in FIG.21).

Although above embodiments of the invention have been described in thecontext of tin, the embodiments may be used for any suitable fuel.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The descriptions above are intended to beillustrative, not limiting. Thus it will be apparent to one skilled inthe art that modifications may be made to the invention as describedwithout departing from the scope of the claims set out below.

It is to be appreciated that the Detailed Description section, and notthe Summary and Abstract sections, is intended to be used to interpretthe claims. The Summary and Abstract sections may set forth one or morebut not all exemplary embodiments of the present invention ascontemplated by the inventor(s), and thus, are not intended to limit thepresent invention and the appended claims in any way.

The present invention has been described above with the aid offunctional building blocks illustrating the implementation of specifiedfunctions and relationships thereof. The boundaries of these functionalbuilding blocks have been arbitrarily defined herein for the convenienceof the description. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description and not of limitation, suchthat the terminology or phraseology of the present specification is tobe interpreted by the skilled artisan in light of the teachings andguidance.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and clauses and theirequivalents.

Clauses

1. A radiation source comprising a fuel source configured to deliverfuel to a location from which the fuel emits EUV radiation, wherein theradiation source further comprises an immobile fuel debris receivingsurface provided with a plurality of grooves, the grooves havingorientations which are arranged to direct the flow of liquid fuel underthe influence of gravity in one or more desired directions.

2. The radiation source of clause 1, wherein the fuel debris receivingsurface is provided with a plurality of vanes, and the plurality ofgrooves are provided in the vanes.

3. The radiation source of clause 1, wherein at least some of thegrooves have a cross-sectional size and/or shape which gives rise tocapillary action.

4. The radiation source of clause 1, wherein at least some of thegrooves have a cross-sectional size and/or shape which gives rise towicking action which draws liquid fuel into the grooves.

5. The radiation source of clause 1, wherein one or more of the groovesinclude a corner which extends longitudinally along the groove.

6. The radiation source of clause 1, wherein one or more of the groovesis v-shaped in cross-section.

7. The radiation source of clause 6, wherein the v-shaped groove has anopening angle which is between around 30° and 50°.

8. The radiation source of clause 1, wherein the grooves comprise a setof grooves which extend substantially parallel to one another.

9. The radiation source of clause 1, wherein at least some of thegrooves have a depth of 0.1 mm or more.

10. The radiation source of clause 1, wherein at least some of thegrooves have a depth of 2 mm or less.

11. The radiation source of clause 1, wherein the at least some of thegrooves have a width of 0.1 mm or more.

12. The radiation source of clause 1, wherein at least some of thegrooves have a width of 10 mm or less.

13. The radiation source of clause 1, wherein adjacent grooves areseparated by a distance which is equal to or less than twice thecapillary length of the liquid fuel.

14. The radiation source of clause 2, wherein the vanes are distributedaround a housing of the radiation source.

15. The radiation source of clause 2, wherein the vanes are reflectivestructures located in the vicinity of an intermediate focus of theradiation source.

16. The radiation source of clause 2, wherein the vanes are located in afuel catcher of the radiation source.

17. The radiation source of clause 1, wherein the grooves haveorientations which are arranged to direct the flow of liquid fuel underthe influence of gravity to a drain or a gutter.

18. The radiation source of clause 17, wherein the grooves are connectedto the drain or gutter.

19. The radiation source of clause 1, wherein the vanes are heated by aheating apparatus to a temperature which is above the meltingtemperature of the fuel and which is below the evaporation temperatureof the fuel.

20. The radiation source of clause 1, wherein the immobile fuel debrisreceiving surface is a bar which extends across an interior of theradiation source and thereby obscures radiation.

21. The radiation source of clause 20, wherein the bar has anon-horizontal orientation.

22. The radiation source of clause 20, wherein the bar includes alowermost portion which is thermally isolated from the remainder of thebar such that transfer of heat from the lowermost portion to theremainder of the bar is limited.

23. The radiation source of clause 22, wherein the thermal isolation isprovided by a narrow connection between the lowermost portion and theremainder of the bar.

24. The radiation source of clause 22, wherein the thermal isolation issuch that in use the lowermost portion of the bar is heated to atemperature which is above the evaporation temperature of the fuel.

25. An apparatus comprising the radiation source comprising a fuelsource configured to deliver fuel to a location from which the fuelemits EUV radiation, wherein the radiation source further comprises animmobile fuel debris receiving surface provided with a plurality ofgrooves, the grooves having orientations which are arranged to directthe flow of liquid fuel under the influence of gravity in one or moredesired directions.

26. A method of generating EUV radiation using a radiation source, themethod comprising:

delivering fuel to a location at which a plasma which emits EUVradiation is generated using the fuel; and

receiving liquid fuel on an immobile surface of the radiation source,and wherein grooves provided in the immobile surface direct the flow ofliquid fuel under the influence of gravity in one or more desireddirections.

27. A radiation source comprising a fuel source configured to deliverfuel to a location from which the fuel emits EUV radiation, wherein theradiation source further comprises a fuel debris receiving surface and aliquid fuel inlet configured to deliver a flow of liquid fuel onto thefuel debris receiving surface.

28. The radiation source of clause 27, wherein the liquid fuel inlet isconnected to the fuel debris receiving surface.

29. The radiation source of clause 27, wherein the liquid fuel inlet isconfigured to provide a coating of liquid fuel on the fuel debrisreceiving surface.

30. The radiation source of clauses 27, wherein the fuel debrisreceiving surface comprises a plurality of vanes.

31. The radiation source of clause 30, wherein spaces between the vanesare grooves which direct the flow of liquid fuel under the influence ofgravity in one or more desired directions.

32. The radiation source of clause 30, wherein the liquid fuel inletcomprises openings located between the vanes or adjacent to the vanes.

33. The radiation source of clause 30, wherein the liquid fuel inlet isconfigured to deliver liquid fuel at a rate that fills an area at thebase of the vanes to a desired fill level.

34. The radiation source of clause 30, wherein the vanes are shaped togenerate capillary pressure which draws liquid fuel away from tips ofthe vanes.

35. The radiation source of clause 27, wherein the liquid fuel inletcomprises a plurality of openings connected to a conduit.

36. The radiation source of clause 27, wherein the radiation sourcefurther comprises a heater configured to heat the fuel debris receivingsurface to a temperature which is above the melting temperature of thefuel.

37. A method of controlling contamination in a radiation source whichuses fuel to generate EUV radiation, the method comprising:

delivering liquid fuel via an inlet onto a fuel debris receiving surfacesuch that a coating of liquid fuel is maintained on the fuel debrisreceiving surface.

38. An apparatus comprising the radiation source comprising a fuelsource configured to deliver fuel to a location from which the fuelemits EUV radiation, wherein the radiation source further comprises afuel debris receiving surface and a liquid fuel inlet configured todeliver a flow of liquid fuel onto the fuel debris receiving surface.

39. A method of generating EUV radiation using a radiation source, themethod comprising:

delivering fuel to a location at which a plasma which emits EUVradiation is generated using the fuel;

receiving liquid fuel on an immobile surface of the radiation source,and using grooves provided in the immobile surface to direct the flow ofliquid fuel under the influence of gravity in one or more desireddirections.

40. A radiation source comprising:

a fuel source configured to deliver fuel to a location from which thefuel emits EUV radiation, and

a fuel debris receiving surface and a liquid inlet configured to delivera flow of liquid alloy or metal onto the fuel debris receiving surface.

41. The radiation source of clause 40, wherein the liquid inlet isconnected to the fuel debris receiving surface.

42. The radiation source of clause 40, wherein the liquid fuel inlet isconfigured to provide a coating of liquid alloy or metal on the fueldebris receiving surface.

43. The radiation source of clause 40, wherein the fuel debris receivingsurface comprises a plurality of vanes.

44. The radiation source of clause 43, wherein spaces between the vanesare grooves which direct the flow of liquid alloy or metal under theinfluence of gravity in one or more desired directions.

45. The radiation source of clause 43, wherein the liquid inletcomprises openings located between the vanes or adjacent to the vanes.

46. The radiation source of clause 43, wherein the liquid inlet isconfigured to deliver liquid alloy or metal at a rate that fills an areaat the base of the vanes to a desired fill level.

47. The radiation source of clause 43, wherein the vanes are shaped togenerate capillary pressure which draws liquid alloy or metal away fromtips of the vanes.

48. The radiation source of clause 40, wherein the liquid inletcomprises a plurality of openings connected to a conduit.

49. The radiation source of clause 40, wherein the liquid alloy or metalis liquid fuel.

50. The radiation source of clause 49, wherein the radiation sourcefurther comprises a heater configured to heat the fuel debris receivingsurface to a temperature which is above the melting temperature of thefuel.

51. The radiation source of clause 40, wherein the liquid inletcomprises a porous metal through which the metal or alloy is delivered.

52. The radiation source of clause 40, wherein the liquid inlet isconfigured to deliver a metal or alloy which is liquid at roomtemperature onto the fuel debris receiving surface.

53. The radiation source of clause 52, wherein the metal or alloy isGalinstan.

54. The radiation source of clause 52, wherein the radiation sourcefurther comprises a cooling apparatus configured to cool a housing ofthe radiation source.

55. The radiation source of clause 54, wherein the cooling apparatus isconfigured to cool the housing of the radiation source to around roomtemperature.

56. A method of controlling contamination in a radiation source whichuses fuel to generate EUV radiation, the method comprising:

delivering liquid alloy or metal via an inlet onto a fuel debrisreceiving surface such that a coating of liquid alloy or metal ismaintained on the fuel debris receiving surface.

57. The method of clause 56, wherein the alloy or metal is deliveredcontinuously via the inlet.

58. The method of clause 56, wherein the alloy or metal is deliveredintermittently via the inlet.

59. The method of clause 56, wherein the inlet comprises a porous metalthrough which the metal or alloy is delivered.

60. The method of clause 56, wherein the metal or alloy is liquid fuel.

61. The method of clause 56, wherein the metal or alloy is liquid atroom temperature.

62. The method of clause 61, wherein the metal or alloy is Galinstan.

63. The method of clause 61, wherein the method further comprisescooling a housing of the radiation source to room temperature.

64. A liquid fuel debris guiding apparatus comprising:

a surface;

two electrodes separated from the surface by an insulating layer, a gapbeing provided between the two electrodes that defines a path on thesurface; and

a voltage source configured to apply a voltage to one of the electrodes,thereby establishing a potential difference across the gap between theelectrodes, the potential difference acting to guide liquid fueldroplets along the path defined by the gap.

65. The liquid fuel debris guiding apparatus of clause 64, wherein theapparatus further comprises one or more additional electrodes connectedto one or more voltage sources, gaps being provided between theelectrodes which define paths on the surface.

66. A method of directing a flow of liquid fuel debris, the methodcomprising:

applying a voltage to one of two electrodes which are separated from thesurface by an insulating layer, a gap being provided between the twoelectrodes which defines a path on the surface, the voltage establishinga potential difference across the gap between the electrodes that actsto guide liquid fuel droplets along the path defined by the gap.

67. A fuel collector for an EUV radiation source, the fuel collectorcomprising:

a receptacle; and

a reservoir, the reservoir being located above the receptacle, and

wherein the reservoir is provided with a hole through which liquid fuelmay drain from the reservoir into the receptacle, and

wherein a raised lip extends around the hole, the raised lip preventingliquid fuel from passing into the hole until a level of the liquid fuelexceeds the height of the raised lip.

68. The fuel collector of clause 67, wherein the raised lip is formedfrom a non-wetting material.

69. The fuel collector of clause 67, wherein the raised lip is formedfrom molybdenum.

70. The fuel collector of clause 67, wherein the raised lip has arounded upper surface.

71. The fuel collector of clause 67, wherein the height of raised lip isequal to or greater than a capillary length of the liquid fuel.

72. The fuel collector of clause 67, further comprising a lip whichprojects downwardly from the hole.

73. The fuel collector of clause 72, wherein the downwardly projectinglip has sharp inner corner.

74. A radiation source configured to deliver fuel to a location fromwhich the fuel emits EUV radiation, wherein the radiation sourcecomprises:

a fuel debris receiving surface; and

a fuel collector, the fuel collector comprising a receptacle and areservoir, the reservoir being located above the receptacle, and

wherein the reservoir is provided with a hole through which liquid fuelmay drain from the reservoir into the receptacle, and

wherein a raised lip extends around the hole, the raised lip preventingliquid fuel from passing into the hole until a level of the liquid fuelexceeds the height of the raised lip.

75. A radiation source housing apparatus comprising:

a rotatably mounted housing;

an actuator arranged to drive the housing to rotate;

a heater located adjacent to a first portion of the housing; and

a cooler located adjacent to a second different portion of the housing.

76. The radiation source housing apparatus of clause 75, wherein thefirst portion of the housing does not have a downwardly facing innersurface.

77. The radiation source housing apparatus of clause 75, wherein thefirst portion of the housing does not have an inner surface from withinwhich liquid fuel can drip.

78. The radiation source housing apparatus of clause 75, wherein thefirst portion of the housing is a lowermost side of the rotatablymounted housing.

79. The radiation source housing apparatus of clause 75, wherein theheater is arranged to heat the first portion of the rotatably mountedhousing to a temperature which is above the melting temperature of tin,and wherein the cooler is arranged to cool the second portion of therotatably mounted housing to a temperature which is below the meltingtemperature of tin.

80. The radiation source housing apparatus of clause 75, wherein thecooler extends around at least two thirds of the circumference of therotatably mounted housing.

81. The radiation source housing apparatus of clause 75, wherein theheater extends around less than one third of the circumference of therotatably mounted housing.

82. The radiation source housing apparatus of clause 75, wherein theheater and the cooler do not overlap around the circumference of therotatably mounted housing.

83. The radiation source housing apparatus of clause 75, wherein theheater does not extend so far around the housing circumference that thetin will remain in liquid form after the housing has rotated to a pointat which the tin is on a downwardly facing surface.

84. The radiation source housing apparatus of clause 75, wherein theinner surface of the housing is provided with grooves.

85. A radiation source comprising:

a fuel source configured to deliver fuel to a location from which thefuel emits EUV radiation; and

a radiation source housing apparatus comprising

a rotatably mounted housing,

an actuator arranged to drive the housing to rotate,

a heater located adjacent to a first portion of the housing, and

a cooler located adjacent to a second different portion of the housing.

86. The radiation source of clause 85, wherein the heater is arranged toheat the first portion of the rotatably mounted housing to a temperaturewhich is above the melting temperature of the fuel, and wherein thecooler is arranged to cool the second portion of the rotatably mountedhousing to a temperature which is below the melting temperature of thefuel.

87. The radiation source of clause 85, wherein the heater is arranged toheat the first portion of the rotatably mounted housing to a temperatureat which fuel on the housing will be in a liquid state, and wherein thecooler is arranged to cool the second portion of the rotatably mountedhousing to a temperature at which fuel on the housing will be in a solidstate.

88. A method of generating EUV radiation comprising:

delivering fuel to a location from which the fuel emits EUV radiation;

driving a radiation source housing to rotate;

heating a portion of the housing to a temperature which is above themelting temperature of the fuel; and

cooling a portion of the housing to a temperature which is below themelting temperature of the fuel.

89. A fuel collector for an EUV radiation source, the fuel collectorcomprising:

a receptacle, said receptacle being provided with an entrance and astorage portion;

an object which is disposed within the receptacle such that fuel passingthrough the entrance is incident upon a surface of the object; and

a fuel transferring mechanism configured to transfer fuel collected uponthe surface to the storage portion.

90. The fuel collector of clause 89, wherein the object is formed from amaterial which is low wetting with respect to a fuel it is desired tocollect.

91. The fuel collector of clause 90, wherein the object is formed frommolybdenum.

92. The fuel collector of any of clauses 89 to 90, wherein the object isoperable to move between a first position and a second position.

93. The fuel collector of clause 92, wherein movement of the objectbetween the first and second positions provides the fuel transferringmechanism.

94. The fuel collector of clause 92 wherein the object is arranged sothat as fuel accumulates upon the surface, the object moves from thefirst position towards the second position.

95. The fuel collector of clause 92, wherein the object is resilientlybiased towards the first position.

96. The fuel collector of any of clauses 89 to 91, wherein the objectcomprises a cantilever structure.

97. The fuel collector of any of clauses 89 to 91, wherein the objectcomprises a wheel and wherein rotation of the wheel provides the fueltransferring mechanism.

98. The fuel collector of any of clauses 89 to 91, wherein the objectcomprises a shelf, and wherein a member that is operable to sweep acrossthe shelf provides the fuel transferring mechanism.

99. The fuel collector of any of clauses 89 to 91, wherein the fuelcollector further comprises first and second valves arranged to form anair lock.

100. A fuel collector for an EUV radiation source, the fuel collectorcomprising: a receptacle, said receptacle being provided with a surfacearranged such that fuel passing through an entrance of the receptacle isincident thereupon, wherein the surface is formed from a material whichis low wetting with respect to a fuel it is desired to collect, thetemperature of the surface is below the melting point of the fuel andthe surface is inclined with respect to horizontal such that the surfaceforms a slide.

101. The fuel collector of clause 100, wherein the fuel collectorcomprises a first valve which is disposed towards an entrance end of thereceptacle.

102. The fuel collector of clause 100 or clause 101, wherein thereceptacle comprises an exit located in a lower portion thereof.

103. The fuel collector of clause 100 or clause 101, wherein fuelcollector comprises a second valve which is located at an exit of thereceptacle.

104. The fuel collector of clause 101, wherein the fuel collectorcomprises a second valve which is located at an exit of the receptacle,and wherein the first and second valves form an air lock.

The invention claimed is:
 1. A radiation source comprising: a fuelsource configured to deliver fuel to a location from which the fuelemits EUV radiation; and an immobile fuel debris receiving surfaceprovided with a plurality of vanes and a plurality of grooves, thegrooves being arranged to direct a flow of liquid fuel under theinfluence of gravity along a same plane towards a drain or gutter,wherein the plurality of grooves are provided in the vanes, and whereinthe vanes are reflective structures located in the vicinity of anintermediate focus of the radiation source.
 2. The radiation source ofclaim 1, wherein the immobile fuel debris receiving surface is providedwith a plurality of vanes, and wherein the plurality of grooves areprovided in the vanes.
 3. The radiation source of claim 1, wherein atleast some of the grooves have a cross-sectional size and/or shape thatgives rise to capillary action.
 4. The radiation source of claim 1,wherein at least some of the grooves have a cross-sectional size and/orshape that gives rise to wicking action which draws liquid fuel into thegrooves.
 5. The radiation source of claim 1, wherein the groovescomprise a set of grooves that extend substantially parallel to oneanother.
 6. The radiation source of claim 1, wherein adjacent vanes areseparated by a distance that is equal to or less than six millimeters.7. The radiation source of claim 1, wherein the vanes are heated by aheating apparatus to a temperature that is above the melting temperatureof the fuel and below the evaporation temperature of the fuel.
 8. Anapparatus comprising a radiation source having a fuel source configuredto deliver fuel to a location from which the fuel emits EUV radiation,wherein the radiation source further comprises an immobile fuel debrisreceiving surface provided with a plurality of vanes and a plurality ofgrooves, the grooves being arranged to direct a flow of liquid fuelunder the influence of gravity along a same plane towards a drain orgutter, wherein the plurality of grooves are provided in the vanes, andwherein the vanes are reflective structures located in the vicinity ofan intermediate focus of the radiation source.
 9. A method of generatingEUV radiation using a radiation source, the method comprising:delivering fuel to a location at which a plasma that emits EUV radiationis generated using the fuel; and receiving liquid fuel on an immobilesurface of the radiation source, wherein grooves provided in a pluralityof vanes on the immobile surface direct the flow of liquid fuel underthe influence of gravity along a same plane towards a drain or gutter,wherein the vanes are reflective structures located in the vicinity ofan intermediate focus of the radiation source.
 10. A radiation sourcecomprising: a fuel source configured to deliver fuel to a location fromwhich the fuel emits EUV radiation; a fuel debris receiving surface; anda conduit connected to a source of liquid fuel and configured to deliverthe liquid fuel from the source via openings through a wall of theconduit directly onto the fuel debris receiving surface disposed outsideof the conduit, such that a coating of liquid fuel is maintained andcontinuously flows across the fuel debris receiving surface.
 11. Theradiation source of claim 10, wherein the fuel debris receiving surfacecomprises a plurality of vanes.
 12. A method of controllingcontamination in a radiation source that uses fuel to generate EUVradiation, the method comprising: delivering liquid fuel from a sourceconnected to a conduit directly onto a fuel debris receiving surface viaopenings through a wall of the conduit, such that a coating of liquidfuel is maintained and continuously flows across the fuel debrisreceiving surface, wherein the fuel debris receiving surface is disposedoutside of the conduit.
 13. A method of generating EUV radiation using aradiation source, the method comprising: delivering fuel to a locationat which a plasma that emits EUV radiation is generated using the fuel;receiving liquid fuel on an immobile surface of the radiation source,and using grooves provided in a plurality of vanes on the immobilesurface to direct a flow of the liquid fuel under the influence ofgravity along a same plane towards a drain or gutter, wherein the vanesare reflective structures located in the vicinity of an intermediatefocus of the radiation source.
 14. A radiation source comprising: a fuelsource configured to deliver fuel to a location from which the fuelemits EUV radiation; a fuel debris receiving surface; and a conduitconnected to a source of liquid alloy or metal and configured to deliverthe liquid alloy or metal from the source via openings through a wall ofthe conduit directly onto the fuel debris receiving surface disposedoutside of the conduit, such that a coating of the liquid alloy or metalis maintained and continuously flows across the fuel debris receivingsurface at room temperature.